Did you use the bathroom at all before you weighed in the 2nd time? You could have lost a little more then.
As far as the exercise goes, I see that all the time. All I can think of is water, (sweat) is heavy. On the days that I do Tae Bo, which really make me sweat, my scale can show a 2 pound drop, however, when I go for a walk and don't sweat much at all, i might show a half pound drop.
I just make sure I never count my weight after exercise. I'll just weigh myself because I'm curious! I only log the weight that I get first thing in the morning.