Why does a car heater blow cool air when the accelerator pedal is not depressed?

my theory is when u accellerate, ur water pupm pumps the water round ur engine at a higher speed than normal. as ur engine is just "tickin over" the pump is still pumpin water round the engine but the engine is working less so produces less heat therefore, water cools by the time it gets to heater matrix as it flows slower. i might be wrong but that's the theory i have had for over a year n im yet to see a different explanation to why it happens.