I think distrusting large corporations by default is a good rule of thumb.

MONEY, DEAR BOY! In having unusual sizes, it forces the customer to try on more things to find the ones that fit. The hope is that they'll buy more than one. If they take it home and it doesn't fit, there's a decent chance that they won't return it for whatever reason so they keep the profits.

At least, that's my guess. That said, I am extremely biased against corporate entities, so keep that in mind.