The main reason is that at the beginning of exercise, the body changes from inactive state to active state. Its physiology, psychology and other aspects have a process of adaptation. Especially people with high body fat rate. Because of its high fat content and relatively low muscle content, people with high weight (especially heavy fat) need to supplement some muscles to adapt to the new exercise environment after starting exercise. At the same time, due to the high weight before exercise and lack of exercise, its exercise itself is equivalent to weight-bearing aerobic exercise including strength exercise. So the increase of muscle weight is greater than the decrease of fat weight. Reflected as the weight loss is not obvious, or even slightly increased.
The platform period of this stage can be successfully spent in about 2 to 3 days as long as you keep a normal mind, continue to exercise and ignore weight changes.