The stats are saying we have a normal distribution (good) but that the middle is L? That doesn't make sense. Large should be larger than a medium! Also doesn't explain why all the good bargain bikes are always in other sizes.
It all depends on the industry trend / ego puffing strategy.
Nobody wants to be thought of as a small person, everyone wants to be a large person. So, for that reason they size L bikes for the most commonly sized person which makes more people feel better about themselves being 'bigger' than they really are.
TLDR:
* L bikes are ridden by median sized folks
* M bikes are ridden by little people who don't want to be labelled little by a whole bike industry
* XL bikes are ridden by lords