I was sorting through some old books and noticed that both these “My little pony” picture books say first edition on them. If they really were first editions they probably wouldn’t be worth much. But, why do they both say first edition, yet only one has the proper “10,9,8,7,6,5,4,3,2,1” printing number indicating a first edition? Whereas the other one says “10,9,8,7,6,5” indicating a 5th edition?
Is this really a first edition? It depends on whether you ask a book collector or a book publisher.
The yellow book you are asking about is the first edition, fifth printing. The purple one is the first edition, first printing. In the second, third, ..., printings, generally the only thing that changes is the printer's key (the line of numbers above).
Book collectors only consider first editions, first printings as valuable "first editions", but for book publishers, a "second edition" is a version of the book which isn't just reprinted, but where something has changed (not necessarily much—possibly a few errors were corrected, possibly just the format—i.e., hardcover or paperback—is new, or possibly it has undergone a very substantial revision).
See this wikipedia page, which says:
Bibliographers usually define a first edition as all printings from substantially the same type setting, no matter how many printings are done. Book collectors tend to define first edition as the first printing of the first edition.