If you drink a pint of water and stand on the scales immediately, your weight will be higher with water in your stomach, but your resistance will not be changed, so your total body water (contained in your muscle tissue) will be low, resulting in a higher fat % reading. (Water is calculated first and the result subtracted from your total weight to get the other readings),
As time goes by, the water filters into muscle tissue. When this happens, the resistance will get lower. The weight will still be high, but the scale will interpret as a higher weight of water. Therefore your readings will show a higher water % and a lower fat %.
If you then empty your bladder, your weight will be lower still, whilst you will still have a high proportion of water in your muscle tissue. This will result in an even higher water % and lower fat %.
This cycle will be repeated naturally throughout the day everyday. However, if anything happens to change the weight or electrical conductivity of the body which is not in this usual daily cycle will affect the readings accordingly. The point to remember is that more water = less impedance = lower body fat % + higher water, FFM and muscle readings.