Brewed a kolsch the other day. Preboil gravity was 1040. However, my post boil was 1080. My burner is like a jet engine and the kettle is a wide 20gal aluminum. Also, I'm only brewing up 5-5.5 gal batches, so, there isn't a lot of liquid in the kettle, relatively speaking. Is it just a matter of vigorous boil off or is there something else amiss?