I have a passing interest in the Septuagint, although I’m not sure the one we have today is completely faithful to the original. I don’t know why the translators would have made that change. The Hebrew can certainly only mean 77, a far cry from 70 x 7 = 490. “Seventy times seven” is also definitely not a typical way for the Hebrew Bible to phrase a number, at least not in the Pentateuch. However, while I was reflecting on it, I wondered if whoever translated it could have been working off an alternate reading of the text.
The verse says:
כִּי שִׁבְעָתַיִם יֻקַּם־קָיִן וְלֶמֶךְ שִׁבְעִים וְשִׁבְעָה׃
Lit: If sevenfold shall-be-avenged Cain, [then] Lemech seventy and seven [shiv’im veshiv’ah].
Now, the second word in the verse is shiv’atayim, sevenfold. What if there existed a version where the last word in the verse mirrored it?
וְלֶמֶךְ שִׁבְעִים שִׁבְעָתַיִם - v’Lemech shiv’im shiv’atayim
“Lemech seventy sevenfold”
That would correspond exactly with the Greek version.
As a matter of faith, I believe in the accuracy of the Masoretic text, but alternate texts abounded during the Second Temple period.