Having done a bit of research, AJ, I can tell you that it is impossible to eliminate all of the variable factors in ecosystems. In fact, an ecosystem is a huge collection of variables. I suspect that one could find a variable in any ecology based PhD dissertation that was overlooked if one tried hard enough.

Hunters themselves are so variable that the only way to make sense out of data is to collect as much as one can and then establish an average. For example, I suspect I am not a typical hunter. I look for the steepest, roughest, hardest to hunt places. I seldom see any other hunters, even on the first day of rifle season. Now, throw in deer, who individually don't behave the way you expect them to all the time. You can see what a challenge the researchers have, and I think they are to be commended for even trying it.