I am saying you cannot use the aggregate sum to judge an individual performance. For instance, in that list were three people who averaged over 30 yards per kick. One that averaged 32 who had the second most attempts. That number, that individual performance, brought up the aggregate number. In fact, his average accounts for around 15% of the average you listed. CLEARLY, the aggregate total you are using to make your argument is skewed more towards his numbers, which are drastically different than anyone else's, as opposed to others.
So yes, your method of calculating the average is completely off base and out of whack. As for the best way to calculate it, I don't know that, but its definitely not the way you did it. Additionally, it would be hard to accurately calculate it because of the modification in kick off rules. One might postulate that the averages have gone down since Hester entered the league because the kickoff units are closer, but Hester's average this year 27.7 was higher than his career average of 24.8. His average since the kickoff rule has been 25.6. So, again, the best way to measure his success is unclear. What is clear though, is that it's not putting more weight on certain returners than other returners averages, especially when those returners averages far exceed everyone else.