Body weight can fluctuate due to water weight fluctuations.
The body regulates its water level depending on its interaction with its environment.
External factors like Weather, Heat, Humidity can result in the body changing its water levels. Internal factors such as digestion, hormonal cascades can also affect the retention of water.
All these can cause body weight to fluctuate across the week.
As long as the body is losing fat, such fluctuations must not be a deterrent for us.