To get an idea, get those new digital weight scales, from costco or bed bath and beyond, they give you a reading of Body Mass Index bmi, water, muscle, bones, and body fat. Oh yes and weight.
From my understanding, they send a little zap through your feet, and calculate.
This way, if you drank a gallon of fluids, and your blood cells are well hydrated, the scale will give you credit for muscle gain and fat loss, even though your gravity weight is up due to water retention and the "output" pending in your intestine.
Read up on BMI its a more honest measurement when dieting and trying to shed fat weight.
By the way, when you exercise, you build muscle and burn fat. Muscle is dense and weighs more than fat.
Example is that your waist line belt line is slimmer (fat loss) but the weight scale says more lbs (muscle gain)
Trust BMI, and get a good bmi scale with good reviews, not the cheapie with sporatic results. Read the manual, for accurate readings, your feet need to be slightly damp or wet for the electric sensor to work.