It's a strange phenomena. Sell content attached to a charity and the Linux users end up the most charitable on average. I don't think the break down of total payments reflects the install base of Windows, Linux & Mac either.
With the Humble Games Bundles, people reasoned that Linux users might be paying more because they were so happy to get games that run on Linux.
The Humble Music Bundle indicates that there is more to it than just supply and demand.
I would guess that a significant percentage of Linux users that buy the Humble Bundles are developers/scientists. These groups create content themselves and therefore they might value content (and especially DRM-free content) higher than other groups.
It might be really interesting if the Humble Bundlers collected some basic demographics.
I suppose Linux users have more money left from not having to buy OS updates every once in a while :P
Jokes aside, Linux users are by definition more attracted to the concept of community and independence. Hence their support to people who believe in the same things.
Why is this?