How does 3500 calories equal a pound... ?

I usually eat 1600 cal a day for maintenance. Rarely, for like special occasions, I eat A LOT and I count it out to be around 2,700, 3000 at the most. However, when I do, I gain 2-2.5 pounds. I thought an excess of 3500 cal is a pound, how is an excess of 1,100-1,400 cal more than even one pound!?