Uninformed guess: When you cut your consumption of calories below your usual baseline, your body adjusts some metabolic parameters so as to use fewer calories (e.g., lower body temperature, less fidgeting, less activity more generally). So there's probably really a range of consumption levels within which, if you consume that much and otherwise do what comes naturally, your weight will be stable. To lose weight, you'll need to cut calories further than you'd otherwise think and/or force yourself to be more active than you feel like being despite the lower intake.