Accepted answer
The short version: reduced maintenance requirement, adaptive thermogenesis and drifting intake, in that order of magnitude.
The correct instrument is a rolling average of daily weights, not a weekly single reading. Daily variation of a kilogram either way is normal and swamps a real weekly change.
More usefully, water retention masks fat loss on a timescale of weeks. Glycogen repletion, sodium changes and menstrual cycle phase can each hide a kilogram or more of genuine loss.
Adaptive thermogenesis beyond that predicted by mass loss is documented in metabolic-ward and doubly labelled water studies and persists for years in some cohorts.
Measure intake honestly for a week before changing anything.