Given the number of such numerically-precise-but-pragmatically-vague sayings in many languages, and the apparent failure of them to converge beyond shared cultural contact (Classical Arabic has the same use pattern for "40", as do many Middle Eastern languages from antiquity, though I'll admit that my linguistic knowledge doesn't do more than touch on this region superficially, other'n a few years of Modern Hebrew), I don't think "arbitrary" quite captures it -- they simply adopted a use pattern that was widespread in the time and place where they were.