With a little bit of calculation, you'll find that the top 10 contributors barely shift the averages at all. Watching the top contributors list early in a bundle lets you see that the platform contribution ratios are pretty well established before the big players come in (I've recorded hourly stats for a couple of bundles which you can view here).

During the HIB V, in response to some comments like these, I calculated out that Notch's $12,345.67 contribution to the Humble Indie Bundle V only shifted the Windows average up by 4c at the time (he selects Windows). Not many of the regular top contributors select Linux anyway.

It'd be nicer to have better indications on Steam itself about DRM status of games (in sales and out of), but there's this community maintained list if that's at all helpful: http://steamdrm.flibitijibibo.com/