About a year ago, I asked similar questions about the 80-20 rule and income distributions. It seems that the simple explanation works surprisingly well. Simulate the following system and it matches real world distributions surprisingly well.

r1) Actors have normally distributed abilities r2) Actors are chosen randomly based on current winnings, the more you have won, the more you compete. r3) Winner of competition wins one point from the loser

You can find what I wrote about this at

http://www.cs.toronto.edu/~arnold/research/80-20/

including the code. It's interesting!