Why does the fan motor go faster and slow at an idle in my 2005 ford escape?

Why does the fan motor go faster and slow at an idle in my 2005 ford escape?

The fan cuts in and out to regulate the engine temperature, based on information sent out by a sensor.

It should run more often when you're not moving, because there's no airflow through the radiator and heat will begin to build up. The faster you go, the less the fan will run, because there's natural airflow through the radiator, so the fan's not needed.