One big factor in return averages is the schedule because returners are pitted against also determine the outcome of a kick/punt return
(kr/pr skill + condition adjust + random) - (opponent run defense + random)
You come up with a value which then determines which bucket of return lengths you will get
Here is an example of some logic for that.
On kick returns if i had things turned all the way up I could notice a guy always being towards the top consistently 20+ yards per return. I'm not sure if it's anecdotal but it seems like more RBs are towards the top of the current season i'm looking at and they tend to have higher rushing numbers than wrs)
On punt returns the averages are so bunched together it we're talking about the top guys at 10-11 yards per return. Not a lot of juice in the squeeze there.