Odd gas guage problem, is god giving me free gas?

I own a 99 Cadillac de ville. I stopped at my girlfriends house and when I came out and started my engine, the gas gauge climbed steadily from 40 liters to 57 over the next 20 minutes. I wasnt parked on a slant or anything and had 30 liters when I stopped. Whats going on here?