Why is the so called Septuagint called “the translation of the seventy” (targum ha-shivi’im) and not “the translation of the seventy two” since 72 (shivi’im ve-shtayim) and not only 70 (shivi’im) sages worked on it?
This is an interesting point.
It seems that there is a difference between the Jewish and the secular opinion on this matter. In Misechet Sofrim (1-7) and Gemorah Megillah 9a it says that there were 72 sages there, while Josephus (chap.17) says that there were only seventy. This is most probably where this term comes from.