Glycogen
Glycogen is the most important “fake fat loss” factor.
It is a long strand of sugar molecules. Your liver and muscles (white muscle cells especially) assemble glycogen for more efficient storage when sugar is excessively available in the bloodstream.
That storage happens rather quickly (unless you’ve developped resistance to insuline, or worse, diabetes), so extra sugar more readily ends up in the liver and muscles than in the fat cells. However, if that storage capacity is full, then the fat cells become the destination of choice.
You store about two pounds of sugar in the form of glycogen. Eat nothing for a few hours and your pancreas will cause the glycogen to be released into the bloodstream (the pancreas releases a hormone called glucagon that causes that to happen).
The glycogen stored in the muscles cannot be released: it is used locally, by the muscle cell that stores it.
After about 24 hours, there is none left and those two pounds are gone. Fat loss does happen simultaneously, but the weight lost in the form of fat is far inferior.