Anyone know why some G1 dolls get major glue-head awhile other G1 dolls don’t?

At some point I plan to go through and clean all my old dolls so I can display them. Right now I just don't have the time or space, so they all just live together in a big box. That being said, I was going through my doll box and wondered to myself why some of my dolls have the awfullest, stickiest glue-heads (like Rochelle) while other dolls still have soft, pristine hair (like Jane Boolittle). If anyone knows why this might be, l'd be very interested to know!