Your progress was 31 not because that was the highest floor you completed before your last reset, but because that's how many floors you completed before resetting. For example, if you completed floors 1, 2, and 30 before resetting, your prestige would be set to 3, not 30. If you did both floors with the same configuration (such as size and complexity), then the amount of prestige experience that you receive will remain constant. The base experience you received for floor 31 would be higher than floor 30. However, there are many other things that could have affected the amount of final total experience you received for that floor, such as not killing all the monsters or not finding all the bonus rooms. It's possible that you killed more monsters and found more bonus rooms on floor 30 than you did on floor 31.