That’s because it comes with a 25% off voucher that works on all the games at their site. In addition, you can trade the game in to get £7/€7/$8 off the purchase price of XCOM: Enemy Unknown (Steamworks).
Note: Unfortunately for me the page got stuck at the processing order page. When I try to buy the free game again it says I already own it. I did not get an order confirmation email like I usually do and it was not in my Spam folder. That means no 25% off code for me. I’ll probably just wait it out and then complain on their forums later. I’ll probably use it on the season pass for Borderlands 2.