In my mind there are two significant reasons which cause the aggregate of skill/talent for professionals to be higher than amatuers.
The average pool of talent is the same whether you're talking about an amatuer sport or a professional one, but the difference is that a profession sport can attract and retain a lot more of the talent beyond the sheer love of the game and against other considerations like education and financial obligations. This doesn't mean that the best talent is unavailable to amatuer competitions and it doesn't mean that all the best talent is available to professional competitions, but what it does mean is that a wider pool of people are willing and able to play and the top flight sports people are selected from that larger, richer group of talent. One example is Cricket, Shane Bond is a truely awesome bowler and he retains to this day one of the best records of any bowler throughout history. Had Cricket not been a professional sport he would still be a policeman.
The second major reason is that more resources can be devoted to developing talent in professional competitions. The time and effort required to lift the performance of a major sportsperson by just 1% is staggering. Not only can the sportsperson devote more time to developing his natural talents but also more resources can be devoted to coaching and other personal development. I'll go back to Shane Bond again. When Shane Bond suffered from repeated back injuries, had he been an amatuer he would have likely quit the sport, whilst as a professional he was able to get the right coaching and medical care to allow him to return to the sport. So whilst hes no longer the best bowler in the world hes still one of the best and that talent would have been lost in an amatuer competition.
Tease.
