septuagint/lxx—why

The Book of the Jonah 1:17, 4:6 comparison of MT and LXX

Septuagint/LXX—why 70 and not 72?

The answer to this might be "go back in time to ask those who said it" but …
There are several documents extant that say 72 scholars produced the Septuagint.  So why is it called Septuagint (for Septuagint, 70) and abbreviated LX…