It's only 10% if you don't consider all the AEGD/GPR residents. I guess they're not as competitive to get into, but that doesn't mean you should count them all out.
You're making the assumption that everyone who has a high board score automatically wants to specialize. Why should that be a goal for everyone who might be capable? Suppose you are the endo master and you can negotiate Z shaped canals better than the endo director. Does that mean you have to be an endo specialist? What if that same person also was awesome at ceramic onlays. Would he have to go prost? If he does either one, he'll have to give up the other. Only a GP gets to do both in his practice and doing a GPR/AEGD might be the route to take.
Why is it hard to believe UCSF would actually want students who like to be GPs? From my limited experience with the school at my interview and speaking with their alumni, I got the sense that there is a definite emphasis on community outreach. As a GP, you'll likely have a larger scope of practice. Jack of all trades but master of none maybe, but at least you can crown your own endo. There's already UCLA to serve the gunner crowd, why not have the other state school be more about serving the community?
UCSF has a number of very high caliber students, much more than 8 per class, so I doubt it's inability to specialize rather than choosing not to that accounts for their low match rate.