I'm one of the lucky few who could always knock out 20 even when I was in highs school. I would have to credit it with doing lat pull downs with more than my body weight for sets of 6-12 reps. It's kinda like the guys in the nfl who bench 225 30+ reps, most train with more than 225 lbs. might have something to do with plateauing to quickly when you only train with 225 or bw for pull ups. I also can do about 25 pull ups with 25 lbs. on my waste now, too bad extra pull ups can't carry over points for my run score.
Pretty sweet dude, I'll be sure to let everyone back in 2008 know the secret to doing 25 pullups with 25 lbs on your "waste."