This happens every bundle. Most people in my experience have concluded that

1. Linux users having fewer games to choose from so they are willing to pay more for whatever games do come out

2. Linux users want Linux to seem like a viable platform for gaming to encourage more game development for Linux

3. There's probably also a competitiveness to seem "better" than Windows users.

See e.g. http://news.ycombinator.com/item?id=2004968