Agree that the difference in weight is caused by fluid retention or fluctuation.

Weight yourself before the ride and again after a ride. On a 4-5 hour ride, if you don't drink enough, you'll lose ~5 pounds. Hydrate correctly and you'll weight the same.

This means the calories and fluids consumed after the ride add to the overall weight. In your case, the extra 2 pounds. This is just normal body fluctuation where the weight difference is more about fluid retention then anything else.