It is designed this way to ensure that a heat surge does not occur in the
coolant system. It puts the fan on when the coolant is hotter than normal,
like after a fast run to the shops without idling the engine before
switching off. When the motor stops there is no cooling airflow through the
radiator and the heat will continue to build so the fan comes on to keep
this heat rise under control. The vast majority of modern vehicles are
designed to do this, particularly compacts and UK & Japanese cars where the
radiator size is limited by underhood space.

All the best, Peter.

700/900/90 Register Keeper,
Volvo Owners Club (UK).