Why? The Windows average is the lowest, meaning that the percentage of people who will buy a game with a fixed price will be the lowest as well. What would be the point of percentage of buyers when it includes all those micropayments? For that to be meaningful, you'd need a cutoff slider, that discards all buyers below the selected amount (now that I think of it, looking at that would be very interesting).
By the way, am I the only one who always thought HumbleBundle stats were misleading in regard to the size of the Linux game market share?
(As in, using amount of payments vs. actual count of payers)