When those two were paying for games it was obvious why that should be, since games on Linux are so comparatively scare, but it doesn't make so much sense with eBooks.
Also, since Windows machines are cheaper, they're more likely to be bought by cheapskates.
On Linux the humble bundle was almost always the first chance to get a hold of the games, while on Windows they'd often all already been out for 6-12 month. I'd almost always already bought and played at least 3 of games in the bundle for $10-20 each and often the games that I hadn't bought where games that I wasn't interested in. So for me 'buying' the Humble bundle was a straight up donation to support a concept I liked, rather than a way to get games. I suspect many windows users where in the same position.
I'm usually around average Linux when I buy a Humble Bundle and I'm making under $50k a year.
I don't have any hard evidence on the success of that, nor do I necessarily feel it is relevant to expand this out to an ebook or music bundle, but the information is there as presented.
(Given that it is not that hard to develop cross-platform especially since OS X is usually targeted as well.)