I am new to brewing(have only brewed three one gallon batches). I did not have the equipment to measure gravity for my first two batches. On my third, I mashed 1.6 lbs grain in 2.5 qts water. After sparging with 1 gallon, My total wort volume collected was about 1.5 gallons. I used an ATC refractometer and got a reading of 1.033, which is what I was shooting for to end up with a post boil OG of 1.05 at 1 gallon. I collected a full gallon, but my post boil OG was 1.04. Did I somehow boil off some of the sugars?