Like anything humans do, their systems and bodies will adapt or attempt to adapt to the behavior. Of course, there will be a huge variation in the ability to do that. I think it was George Benton who mentions how fighters skins adjust to getting hit so that what left a mark before, wouldn't do it as they got used to it. And, the features change, Holyfield is a great example, his nose just got more smushed in and his features were generally just wider and lumpier. Not only may the skull change in it's ability to absorb shock, it changes in shape fairly dramatically over time too, you look at profile pictures of fighters when they are young and when they are at the end of their careers and you'll notice that there is a flatter appearance. A million things a top pro has adapted to that a civilian or even an amateur hasn't. I recall watching an ammie tournament when i was a kid and the first thing that seemed the most different was how everone's nose bled, again, not unheard of in pro boxing but not as common.
As far as bone density, from what I read, the researches described the bones as having spaces in them and that stress makes the spaces fill up, and they compared it to concrete being poured into forms. Now skin? I have heard that Jack Dempsey soaked his face in pickle brine and believed it worked (heard the same for his hands) but I would think that the facial skin would be one of the least adaptive things to getting hit. Ali was known to have skin like leather and was only cut once and maybe twice as a pro while Jerry Quarry cut at the ring introductions.
anyway, i'm always reading about how some minor streetfight ended up with a guy with major facial damage and I know goddamned good an well, the average person really can't hit that hard at all. I've had a friend who had his cheek bone punched in when he was drunk, I doubt the guy that hit him threw a lot of punches before that. I find it very hard to believe that a skull that easily damaged by a nobody, is the same as the skull that pros have. Same with a chin, look on youtube, it's full of guys who fall down as dead when some idiot hits them, you think all those guys have alexis arguello power?