It is impossible for us to say how come you have a different version—we’d have to know how you learned the poem, which you didn’t tell us, and you likely don’t remember. But we can investigate the general problem of corruption of texts.
Fallibility of memory
People are capable of prodigous feats of memorization: the actor playing the lead in Hamlet, for example, has to deliver more than 1,500 lines, and in the theatrical world this is not considered a particularly notable feat. But memory does not work like a camera or printing press: the process of recollection involves a certain amount of reconstruction, and that can result in the introduction of errors. Poetry is more resilient to error than prose because rhythm, rhyme, and imagery help anchor the major words in their places. But the minor words are subject to change, and minor words can change the whole meaning of a passage.
Here’s an example of this process from a notable writer. George Orwell found it easy to memorize poetry:
I know a respectable quantity of Eliot’s earlier work by heart. I did not sit down and learn it, it simply stuck in my mind as any passage of verse is liable to do when it has really rung the bell. Sometimes after only one reading it is possible to remember the whole of a poem of, say, twenty or thirty lines, the act of memory being partly an act of reconstruction.
George Orwell (1942a). ‘Points of View: T. S. Eliot’. Poetry London, October–November 1942. Reprinted in The Collected Essays, Journalism and Letters, volume 2, pp. 272–3. London: Penguin.
This facility of memory must have been a benefit to Orwell when composing his critical essays, because it obviated the need to visit his bookshelves, or the library, find the volume containing the lines to be quoted, and locate them in the text. But how accurate was his memory? Here are a couple of examples, with differences in bold.
The stream of the world has changed its course
And with the stream my thoughts have run
Into some cloudy thunderous spring
That is its mountain source—
Aye, to some frenzy of the mind,
For all that we have done’s undone,
William B. Yeats (1914). The Hour-Glass, p. 34. Privately printed.
The philosopher in the play dies on the knowledge that all his lifetime of thought has been wasted (I am quoting from memory again):
The stream of the world has changed its course
And with the stream my thoughts have run
Into some cloudly, thunderous spring
That is its mountain-source;
Ay, to a frenzy of the mind,
That all that we have done’s undone
George Orwell (1943). ‘W. B. Yeats’. Horizon, January 1943. Reprinted in The Collected Essays, Journalism and Letters, volume 2, pp. 315–316. London: Penguin.
The substitution of ‘that’ for ‘for’ in the last quoted line substantially changes the meaning. In Yeats’s original, the Wise Man says that his thoughts have become frenzied because things have been undone. But Orwell’s version has the causation the other way round: his thoughts have become frenzied so that things can be undone. This better suits his criticism that this passage is “by implication profoundly obscurantist and reactionary”.
I ’eard the knives be’ind me, but I dursn’t face my man,
Nor I don’t know where I went to, ’cause I didn’t ’alt to see,
Till I ’eard a beggar squealin’ out for quarter as ’e ran,
An’ I thought I knew the voice an’—it was me!
Rudyard Kipling (1896). ‘That Day’. In The Seven Seas, p. 180. London: Methuen.
I ’eard the knives be’ind me, but I dursn’t face my man,
Nor I don’t know where I went to, ’cause I didn’t stop to see,
Till I ’eard a beggar squealin’ out for quarter as ’e ran,
An’ I thought I knew the voice an’—it was me!
George Orwell (1942b). ‘Rudyard Kipling’. Horizon, February 1942. Reprinted in The Collected Essays, Journalism and Letters, volume 2, p. 222. London: Penguin.
In this case Orwell doesn’t say explicitly that he is quoting from memory, only that “verse is a mnemonic device, among other things”. But the mistake suggests that he is. Here “stop” alliterates with “see”, and it doesn’t have the dropped aitch of “’alt”, which Orwell finds embarrassing:
Kipling idealizes the army officer, especially the junior officer, and that to an idiotic extent, but the private soldier, though lovable and romantic, has to be a comic. He is always made to speak in a sort of stylized Cockney, not very broad but with all the aitches and final ‘g’s’ carefully omitted. Very often the result is as embarrassing as the humorous recitation at a church social. And this accounts for the curious fact that one can often improve Kipling’s poems, make them less facetious and less blatant, by simply going through them and transplanting them from Cockney into standard speech.
Orwell (1942b).
Examples from Literature Stack Exchange
A look through the story-identification questions on this site will show the operation of memory: questions where minor details have been mis-remembered are the rule rather than the exception.
In this question, lines from Honey Bear by Dixie Willson have been remembered, but “places where the sky looked through” has become “spaces” (logically enough—there must be spaces between the leaves of the trees) and “big old hollow tree” has become “deep” (again, logically enough, the hollow must be deep enough for the bear to live there).
In this question about ‘The Most Dangerous Game’ by Richard Connell, a pipe knocked out of the protagonist’s mouth by a rope has become a cigar knocked out of his hand by a sail—the kind of detail you might come up with if you retained a general impression of the scene and had to reconstruct it.
In this question about ‘Go to the ant’ by Stanley J. Sharpless, “one up on us guys” has become “as good as us guys” (a natural change because “one up on us” is rather awkward, and “good” alliterates with “guys”).
Lectio difficilior
As you can see from the examples above, recollected versions often have features that are ‘better’ (for certain purposes) than the original. Orwell’s recollection of Yeats changes the causation in a way that better supports his criticism of Yeats as reactionary; his recollection of Kipling avoids the embarrassing dropped aitch; the changes to Honey Bear are logical improvements; the change to ‘Go to the ant’ makes the wording smoother and adds an alliteration.
The same kind of ‘improvement’ is going on in the altered version of ‘Stopping by Woods’. It is easier to read the “though” in the second line as referring backwards (“I do know, though, that his house is in the village”) rather than forwards (“he won’t mind me stopping here, though, because his house is in the village”). And if you followed this easier reading then the first line would make more sense if it were contrastive.
However, the fact that the remembered versions are ‘improvements’ on the commonly accepted versions is actually evidence that the remembered versions are the corruptions, because it is more likely that you would make a reading smoother, or easier to understand, or better suited to your purposes, than the other way round. This is the principle of lectio difficilior potior (“the more difficult reading is the stronger”) in textual criticism.
Once an ‘improved’ version becomes established, it persists because it is easier to remember, or makes more sense, than the original.
