Either the mechanism to measure flow through the gas pump was messed up and gave me free gas, or I can’t do math…

My truck holds 32 gallons of gas. I was *very* empty (it would be a stretch if I had 2 gallons in the tank). Anyway, I fill it up and the pump stops at 20 gallons, so I start it again and it immediately stopped again (like the tank was full). So I check the gas gauge in the car and it shows as being full.

So either my gas tank shrunk by 10 gallons, or that gas pump just gave me ~10 gallons of free gas. Weird…