I *think* what happens during a long ride is two-fold. One is simple dehydration, the other is a purge of your body's glycogen.
Glycogen is how your muscles store sugars for quick energy. Each gram of glycogen also stores about four grams of water with it, if I remember my biology right. For about the first hour of a long ride, your body can pull energy from muscle glycogen, which drains some of the water as well, until it switches to the slower burn of the liver-glycogen production, fat-metabolism, or the dreaded ketone metabolism (bonking).
When you're done using your muscles, you'll eat a recovery meal or whatever, and one of the first things that gets replenished is that glycogen and the water that goes with it. It's like a sponge that soaks up water and stores it back into those muscles for future use, sometimes a little more than you may have stored before the ride.
I remember in the old days when I was playing around with carbohydrate cycling and heavy weight lifting, I could get my weight to swing six pounds in a day, and a glycogen refeed would cause my arm muscles to *pop* and the scale would skyrocket. What you're seeing is probably not as quick or dramatic, but it might be the same issue. Either way, it's not a reflection on true fat-loss or -gain, unless somehow you're taking in WAY more cookies and burritos on the ride than you're burning. Then you're on your own. :-D
-- gnat!
