Eats X amount of calories consistently for weeks - weight and fat loss stagnate. Eats X + 250kcal for a few days - weight and fat loss have started up again. Just as people overestimate their maintenance calories and end up eating too much, so do other people underestimate it and eat too little. If something's not working, make small adjustments, observe, document and experiment. Even if your calories per day are a magic number, a lie, it can be tweaked consistently to arrive at the truth.