Some of it is genetics and luck, some of it is from sparring. Terry Norris didn't take too many prolonged beatings in the ring for a fighter of his experience, his chin didn't really allow it, but he was known to go all out in sparring.
Because Toney fought in the pocket most of his career, and even though he did'nt seem to get hit cleanly that much, he still took a fair amount of short choppy punches inside imo, not to mention the amount of fights he's had. Frazier is pretty self explanatory, he was a guy that did'nt really jab that much, preferring to close distance fast by roll into position then trying to work you over inside. Anyone who closes distance that fast, without a jab, whether there head is in perpetual motion or not, is going to get hit on the way in regardless, more so when they always want to fight inside. Whitaker fought in the pocket most of the time to, he also had over 200 amateur fights. The Whitaker we saw against Nelson, DLH, Chavez is not the stereotypical way he fought imo, its just perceived that way cause they were two of his bigger fights, so when someone wants to see what all the fuss was about regarding him, they watch these fights, and auto assume he was a runner because he fought exclusively from long range in those fights and used his legs.
