I don't understand why some do and some don't but think it's probably not cause for a great deal of concern ... embarassment sometimes, yes. My little angel brat, of course, does not do it. If she did, we would just try to find a correction for it.
We bought Toto one of those long legged stuffed toys way back when we were learning about her and it was about three times her size but she loved it and would play with it and would not sleep without it. It had a mouse looking face and was a peach color ... we named him "Chewie". She played with it for years and then one day her BFF, Shira, was visiting ... yep, Shira humped Chewie!
Poor Toto was mortified and never touched her beloved "pet" again!!