The Humble Weekly Sale: Nordic Games has been launched for some time and it's available only for the Windows platform. This didn't stop the Linux users from purchasing the collection.
The Humble Bundle initiative has provided some great deals and Linux games in the past, but they also release Windows only games from time to time.
This is just the case with the Humble Weekly Sale: Nordic Games which includes Supreme Commander, Supreme Commander: Forged Alliance, The Guild 2, Red Faction: Armageddon, Painkiller: Hell & Damnation, ArcaniA, Darksiders II, and SpellForce 2: Faith in Destiny.
None of these games are available on the Linux platform, but the statistics on the official website show that over 5% of purchases have been made by Linux users, or those who have declared themselves as such.
From all the games listed above, only Painkiller: Hell & Damnation will probably feature a Linux version sometime in the future.