I quess there was some disbelief on my part that this thread was actually real, but I quess it is a serious questions. Well, let’s look into it.
First, All variables between weighings must be the same: clothing, posture, position of the scale, etc. Second, for the weighing to valid the scale needs to be very accurate. The normal bathroom scale just isn’t accurate enough to trust a 1 pound difference between weighings. Many of them use some sort of spring or compression mechanism. Third, weighings should be done from an impartial third party. Unless all these conditions are met the differences may not be real differences just indicators there are spurious variables between the weighings.
If the conditions are met and the weights are actually different, there are only a few kinds of inputs and outputs the body can make to change weight. The first has to do with the exchanges of gases, most of which is done through respiration. During and after exercise the blood may have a high concentration of CO2. Your body may also be taking in more O2. I doubt this would make a huge difference in weight, but I could see a small difference.
Second, the intake and output of fluids can affect weight. Fluid intake is primarily through drinking, but there can be some water taken in and let out with respiration. However, since you are not drinking during your workout it is likely the fluid lost would lead to a loss in weight. Unless of course that water is remaining in your clothes or on your skin and is being accounted for in the weighing.
Food intake and calorie burn is also an area where weight can be added or subtracted. I doubt you are eating a lot during a workout. I also doubt you are burning a lot of calories during a workout.
Even a marathon workout session wouldn’t burn no more than a pound or so of calories. If you actually did burn that many calories during a workout session you probably wouldn’t be worried about stepping on a scale.
Other than respiration, fluid intake, anc calorie intake I cannot think of any major inputs/outputs other than urination and defecation. Both would lead to temporary minor weight loss.
The most likely cause of the differences is inconsistencies in the weighings or inaccuracies in your scale.