Why a deficit burns fewer calories than expected
Adaptive thermogenesis is the body's tendency to reduce energy expenditure during sustained dieting by more than the drop predicted from lost body mass alone. As you lose weight you naturally burn a bit less because a smaller body costs less to run, but adaptive thermogenesis is the extra reduction on top of that, an active conservation response to a prolonged energy shortfall.
It helps explain why weight loss often slows over time even when a diet is followed consistently, as the gap between intake and expenditure quietly narrows.
Where the savings come from
The largest contributor is usually a decline in NEAT (non-exercise activity thermogenesis): spontaneous movement like fidgeting, posture, and general daily activity falls, often without conscious awareness. There is also a modest reduction in resting metabolic rate beyond what the lost mass accounts for, and hormonal shifts (such as lower leptin and thyroid signaling) accompany it.
The magnitude varies between individuals and with the depth and duration of the deficit, so any single figure is an average rather than a fixed amount. This is general educational information, not medical advice.