It's because you aren't looking out of the center of your eye, but through the top a little more. I do the same thing but I'm trying to fix it. I feel like a ton of shooting is about how much you see, and how you see. But when I hunch and look out the top of my eyes I don't have as good a field of view. I'm trying to keep my head up but it seems to be something I struggle with.
-Cory