Our bodies are not mostly composed of fat, they’re mostly composed of water. Therefore, most of the weight changes we see from day to day are water changes, not fat changes!
A few reasons you see fluctuations on the scale:
Recent increase in cardio (increases his blood volume). This is a biggie, can add up to 5lbs in some people, just with plasma plus new red blood cells, and can cause a month-long stall in scale weight.
Recent increase in weight-lifting (sore muscles retain water); eventually they will also be muscle gain, though that is a slower process (weeks).
Any soreness, injury or bruising (causes inflammation = local water retention)
Change in timing of dinner to a later time than usual, and/or addition of a late night snack, even if you ate the same total number of daily calories. The timing alone has an effect. Later eating means that the gut will still be holding “gut water” used in the digestive process during the next morning’s weigh in. Even 1 hr later dinner time can affect scale weight the next morning.
One unusually large meal within the past five days- large meals take multiple days to digest, during which not only the meal itself, but also the mass of water equivalent to two times the meals weight, will all be parked in your gut for digestion.
Eating more carbs after a period of not having had many, (causes liver and muscles to increase glycogen stores; each 1g of glycogen is structurally packaged with 4g of water so you end up with water weight).
Change in dietary fiber, change in bowel movement frequency,
Recent exposure to heat-hot yoga, exercising in heat!
Menstrual cycle/birth control-about 1/3 of women have noticeable swings in weight week before & during menstruation.
Increase in sodium intake = increase in water!!!
In short, trust your calorie deficit. If you are tracking your food accurately and are really eating at a calorie deficit there is no way NOT to be losing fat. The fat loss is just hidden on the scale by the myriad reason of fluctuations in water and digestive waste! Stay the course!