I have consulted several translations and each of them renders the "proportions" the same, i.e. one third man and two thirds god.
This claim has been translated in slightly different ways.
In Andrew George's translation (Penguin, 1999), we find the following lines
Gilgamesh was his name from the day he was born,
two-thirds of him god and one third human.
(Andrew George relied on the actual tablets for most of his translation, instead of the pen-and-paper drawings provided by other assyriologists.
He also published the currently most authorative edition of the Akkadian texts.)
In Stefam M. Maul's German translation (C. H. Beck, 2005):
"Gilgamesh" ist er seit dem Tage, da er geboren, mit Namen genannt,
Zwei Drittel an ihm sind Gott, doch sein (drittes) Drittel, das ist Mensch.
("(...), / Two thirds of him are god, but his (third) third part, that is human.")
In Wolgang Röllig's German translation (Reclam, 2009):
Den Namen Gilgamesh trägt er seit dem Tag seiner Geburt.
Zwei Drittel von ihm sind Gott, ein Drittel Mensch.
("(...), / Two thirds of him are god, one third human.")
In Benjamin R. Foster's translation (second edition, Norton, 2019):
Gilgamesh was singled out from the day of his birth,
Two-thirds of him was divine, one-third of him was human.
The most interesting difference here is the different tenses that are used in these lines.
The context of these verses is the narrator's account of Gilgamesh' achievements, which, from the narrator's point of view, are situated in the past.
However, the tense in the verse describing the godly and human shares in Gilgamesh is sometimes a past tense (Foster), sometimes a present tense (both German translations), while Andrew George avoids choosing a specific tense.
(Possibly, Akkadian verbs did not have tenses, like Standard Chinese today.)
The present tense can be defended on the ground that Gilgamesh was deified after his death and became one of the judges in the Netherworld (thereby achieving the immortality he had been looking for).
This would assume that Gilgamesh did not lose his human share after his deification.
So is the narrator's description of the godly and human shares based on the later deification of Gilgamesh, or are there any arguments that support the thesis that he is describing the shares as they were before the death of Gilgamesh?
It appears to be the latter, since these shares are also mentioned by the scorpion-man's mate in Tablet IX (in Andrew George's translation):
Two-thirds of him is god, and one third human.
Translators do not always explain the two-thirds-versus-one-third ratio. Andrew George and Benjamin R. Foster do not comment on it at all.
Stefan M. Maul (page 155) points out that Lugalbanda, who is considered Gilgamesh' father, was also deified after his death, so Gilgamesh may have inherited a both a human and a godly part from him. (His is not clear on the "biology": did the union between Lugalbanda and Ninsun take place before or after his death?)
Wolfgang Röllig (page 156) also mentions Lugalbanda's deification, but without mentioning heredity. However, both translators point out in a comment on line 36 that Gilgamesh had drunk milk from Ninsun's breast.
The claim that the ruler had drunk the milk from a goddess's breast was a common motif in Babylonian literature and occasionally also in Babylonian art.
Raoul Schrott's German translation has a footnote for the passage about the divine element in Gilgamesh (footnote 11 on page 178):
Er hat also doppelt soviel Göttliches wie die übrigen Menschen, was den sumerischen Sexagesimalsystem nach der Zahl 60 entspricht - (...).
My translation:
He has twice as much "divinity" as other humans, which corresponds to the Sumerian sexagesimal system, which is based on the number 60 (...).
Genetics are irrelevant here; Schrott also adds that the goddess Ninsun and the king Lugalbanda are Gilgamesh's parents only in name.
However, as far as I know, Raoul Schrott, unlike the other translators listed above, is not as Assyriologist.
Conclusion: None of the translators find the two-thirds-divinne-versus-one-third-human ratio strange in the context of a heroic epic.
However, their explanations don't agree on the cause of this ratio: either humans generally already had a divine part and Gilgamesh just had a bigger share (due to his mother Ninsun),
or Gilgamesh had a larger divine share because his father had been deified.
Perhaps it is not remarkable that the son of a divine mother and a human father is two thirds god because (a) he spent (presumably) nine months in a goddess' womb and (b) was breastfed by his mother, so it is logical that "her share" in him is bigger?
The introduction to Benjamin R. Foster's translation (2nd edition, Norton, 2019) contains a subchapter on "Use of Fantastical Number", which provides the following comment on Gilgamesh's divine-to-human ratio (emphasis mine):
Among the epic's most celebrated riddles is Gilgamesh's genealogy as two-thirds divine and one-third human, for which various explanations have been offered. The fraction two-thirds appears again in the name of the boatman, Ur-Shanabi, "Servant of Two-Thirds", and in connection with completing the Ark (XI, 97).
There is another piece of interesting background information: pages xxxix - xl of Andrew George's introduction also discuss how Ea created man:
The clay that Ea gave to the Mother Goddess as the raw material from which she bore mankind was animated - given spirit - by mixing it with the blood of a god:
Let one god be slaughtered and the gods be thereby cleansed. With his flesh and his blood let the Lady of the Gods mix some clay, so that god and man are mixed together in the clay.
In other words, ordinary human beings also had a divine element. The text does not specify to what proportion humans are "divine".
(See also Chapter II. Myths of Origins in Samuel Noah Kramer's Sumerian Mythology, which mentions that the blood was "of one of the more troublesome of the gods".)
