Quality control would be my guess as well.
Kenner even has numbers & letters on the inside of the torso's, if you split the torso apart. I would assume, since LL got some of the old Kenner moulds, you would find the same letters and numbers on the inside torso's. I haven't been brave enough to split a beater LL apart yet, to check.
Anyone with overstock LL non sonic welded figures would be able to confirm.