What's going on with the gas mileage in my car?

My 2006 Hyundai Sonata is telling me I am getting 21 miles to the gallon, which isn't bad. But I filled up on Monday, and now I only have a quarter of a tank left. I don't think I am driving enough to use fuel that quickly, and I have a lock on my gas tank door, so nobody could be siphoning it. Does anybody have any idea what the deal might be?