The Septuagint derives its name (Latin septuaginta, 70, hence the abbreviation LXX) from a legendary account in the Letter of Aristeas of how seventy-two Jewish scholars (six scribes from each of the twelve tribes) were asked by the Egyptian pharaoh in the 3rd century BCE to translate the Torah for inclusion in the Library of Alexandria. Although they were kept in separate chambers, they all produced identical versions of the text in seventy-two days. Although this story is widely viewed as implausible today, it underlines the authority that the translation had among Jews.But looking briefly at a translation of the Letter of Aristeas I can't find anything that describes how all seventy-two scholars miraculously came up with the same translation of the whole text; it seems like the letter might just be talking about a task divided among all seventy-two coincidentally happening in seventy-two days. Not that that necessarily has the ring of historical truth about it either, but it's not as cosmically implausible as an identical seventy-tuplet of independently translated copies of the OT showing up.
Also, why the heck is it the Septuagint if all the numerology points to 72, not 70?