I hope that steam breaks the myth that Linux users will not pay for software.
That said, I'm not sure how widespread this myth really is outside of select Internet communities. Enterprise Linux users are known to pay for their software (even if somewhat indirectly, e.g., through support contracts), and the biggest myth concerning desktop Linux users seems to be "huh, are there any?".
[1] See http://cheesetalks.twolofbees.com/humble/ for a great report.
Even if you ignore the overrepresentation of Linux users, the fact stands that when you look at total revenues, Linux rivals OS X.
We know that the HIB has drawn in significant revenue from Linux customers, but that doesn't give us the faintest clue how profitable they are (remember, that average $11 Linux sale has to be shared with multiple developers and charities). You might take the fact that they keep offering more HIBs as a sign that Humble Bundle (the company) considers its humble bundles (the promotions) to be successes, but it's not a guarantee that the metric they're using to define success is profitability. They've raised nearly $4.5 million in funding, so they're not strapped for cash; it's very possible they're more focused on using the bundles as a marketing channel than as an actual revenue stream, convincing consumers to trust the Humble Bundle brand name and developers/publishers that they're a meaningful distribution platform.
tl;dr: the fact that a successful venture-funded startup is selling software to Linux users, when we don't know if they're profitable or what their long-term strategy is, doesn't necessarily imply that there's a viable market for selling software to Linux users.
You also have to note that while Windows users are more likely to own the games prior to any given Humble Bundle sale (esp. given that many games were first ported to Linux specifically for a Bundle) at the time of the sale they are unlikely to find a better deal on any of the games either separately or as a package, even games unlocked for beating the average payment. That would make buying those games from the Bundle a rational choice if you are a Windows user and want even one of them.
Of course, the above argument doesn't negates the fact that Windows gamers have more choice. For them the opportunity cost of paying more for a HIB could be not buying another game on Steam, GOG or elsewhere.
I guess in the end it all hinges on how much the buyers (all of them: Windows, Linux and Mac users) are interested in playing those games specifically vs. playing any games at all. If they are interested specifically in the titles sold, then yes, the statistics would mean that Linux users are more generous. Otherwise it's probably more about opportunity cost.
I personally bought the bundles for Windows, but put in Linux and Windows for the survey, just for an attempt to make a tiny dent in the stats.