Body fat percentage measurements with home scales are not very accurate. It gets better if you have a four-point scale, whereby you need to hold a handle with both hands. But it usually only has electrical contact pads at your feet. An electrical current goes in at one foot, and come out at the other. This is used to measure the impedance, and to derive the body fat percentage from. However, the electrical current will take the shortest path and will thus not go through your upper body. If the body composition of your upper body changes, changes are that this is not detected by the measurement method.
This is why a four-point measurement is a bit better as it also sends electrical currents through your arms. It can measure the path from one arm to your feet, thus including your upper body.