I've wondered that too, but I think it may be along the same lines of competition shooting gettin you kilt in the streetz. It could be the difference between a guy who grew up street tough and got into fighting/martial arts later vs a guy that grew up wearing polo shirts and doing point karate.
Out of the guys at my gym who are either amateur or pro fighters, only one of them that I know of has been in a street fight since I've known him and he fuuuuuuucked the guy up. I know the guy he beat up(small town) and the dude has been fighting his whole life. It might be one of those things where "it depends" is the right answer.