How is the Normandy refueled?
I've never bought fuel at a fuel depot, but I sometimes find the Normandy has returned to 1000 fuel. I've found fuel a few times while scanning, but definitely not that much. Are there other actions that cause the Normandy to refuel?
Docking at the Citadel gives you a refueling. If you've ever gone to the Citadel, that'd be why you have your fuel back.