I think you're right, certainly a lot of factors come into it. How often your opponent is at net must be a huge factor; so I've got some data on how many of those winners were passes, but that data is hard to come by. Usually we have to count it ourselves.
Another factor is how many times the net-rusher approaches to a particular side.
Certainly surface is a huge factor, and technology.
And as you mentioned, a comparison against the errors is another important factor. But that one, with the published stats, is a problem because immediately you have to consider the method of counting unforced errors. Who was the statistician, how did s/he count, etc. That would get especially tricky comparing UE stats across eras; that's a huge problem. Sometimes I see comparisons where one players in one decade "had" X number of UE's, while another in this decade "had" another number. But there's nothing objective about UE's. Rather than saying, "he had" a certain number, it's probably more accurate to say, "he was given" a certain number.
At least winners are objective, when we count them ourselves; and when I include published stats I try to stay away from numbers that for one reason or another look inflated (ie, with lots of judgment calls in the winner column).
I think of these charts as giving a snapshot of what some famous players have done with winners; nothing more or less. From there I think it's possible to say this or that about the numbers, and to start debates. But I think it's going to be difficult to find anything conclusive in these numbers.
Agree with everything you've wrote. I like your charts for winners in this thread. It's very interesting and of course informative.