Glycogen is the storage form of carbohydrates in your body. It sits in the liver and, above all, in your muscles, and serves as quickly available fuel, for example for high-volume lifting. Training uses some of it up, and afterwards your body refills the stores.
Glycogen binds water
Your body does not store glycogen dry but together with water. Kreitzman and colleagues (1992) give three to four parts water per part glycogen. A study with muscle biopsies (Fernández-Elías et al. 2015) found at least three grams of water per gram of glycogen.
Example: If your glycogen stores change by 300 g, 900 to 1,200 g of water come with it. In total the scale moves by about 1.2 to 1.5 kg without a single gram of fat. These numbers are a worked example, not a prediction for you.
How big the stores are depends on body size, muscle mass, training and diet. In an overfeeding experiment (Acheson et al. 1988), participants with depleted stores were able to take up about 500 g of extra glycogen.
What this means for bulking and cutting
When you start a cut with fewer carbs, your stores empty and the first week shows a loss well beyond your deficit. Switching to a bulk, a refeed or a diet break does the opposite. After a hard workout or a cheat day, the scale often shows more too, because the stores are filling up.
These water shifts show up in your trend as a step or a short spike, not as a lasting slope. So only judge your rate once the step has settled, usually from the third week of a new phase.