Why do some Barbie Dolls have a different date on the back of their head than on their body, other than people may have changed the head obviously?

The body shape was copyrighted at a different time than the head shape or style. The body is basically the same no matter what Barbie you buy, with certain exceptions, of course, but they are different dolls depending on the style of head (color of hair, makeup, smile, etc.) The head style will be copyrighted when it is designed, and put on a body style that may have been copyrighted years earlier.

https://www.dollreference.com/barbie-vintage-dolls-identified-1959-62/
https://www.reddit.com/r/Barbie/comments/194hxdr/copyright_dates/