KF Killing Floor is the ONLY fps I don't get nausea playing, any ideas why?

Thinking it was an FOV-related issue, I researched Killing Floor's default FOV value (85) and changed Dead Island's and Half-Life 2's FOV values to 85.



Still, I got nausea.



So I went further and even turned off bloom and motion blur.



But still, I get nausea.



Why??



What is it about Killing Floor that I can play it for a whole day and feel completely fine, but I play Half-Life 2 or Dead Island for FIVE (YES, FIVE) FREAKIN' SECONDS (COMPLETELY SERIOUS, NO JOKE) and instantly get nauseous?



Please! Someone shed some light on this! My entire gaming future depends on this!!