Looking at my (non-split) keyboard, the 6 is clearly more on the left hand side. The center of the home row, between G and H on a Qwerty keyboard, just about lines up with the right edge of the key cap, and it is less of a stretch to press it using the left hand. If a keyboard is designed to be ergonomic, it might be decided that you shouldn't need to unnaturally stretch to reach the 6 key using your right hand, so placing it on the left arguably makes sense. I suppose if one is entering a bunch of numbers, that could lead to some additional movement on the left hand side, though it's probably better to use a numpad for heavy number input anyway. Also, who is to say that Mavis Beacon is "correct" on the matter? Some other typing resources have assigned the key to the left hand, even if it tends to be less common.
Of course, when it comes down to it, the entire qwerty/azerty layout is arguably a mess, designed around reducing typebar jams on old mechanical typewriters, rather than anything resembling efficiency or ergonomics. It doesn't really make sense on computers, aside from not requiring those who learned to type on a typewriter to learn something new, and now we're stuck with it for pretty much the same reason.