Why do i weigh less at the gym then at home?

when i got up this morning i weighed myself and was 115 pounds then i had a bagel with cream cheese and went to the gym. before my workout i decieded to use one of the scales at the gym and i was 113.7 pounds. why is this? i had eaten and not worked out yet but weighed less?? which scale should i trust (home or gym)??