This is a really interesting statistic actually. I got the last 2 bundles and I'm a Mac user. I am definitely pro-Mac OS. Seeing those stats made me pay a little more.
I am also an iOS dev and try and keep up on trends. I'd wager most Mac users are also iOS users. And current stats indicate that most iOS users are willing to spend money on software.
iOS users get many chances to spend money on games. Linux users only get a few chances, so when a Humble Bundle rolls around, they jump on the opportunity. You can bet that iOS users spend more money (total, not per-game) on games than Linux users, because of the App Store.
I guess Mac users spend all their money on OS upgrades :)