Heater Auto setting and defrost fan?

I understand that when I turn the auto setting on, the heat often comes out from the bottom and also from the defrost vent as the front window frequently fogs. I've watched and as the interior of the car warms, the defrost fan goes off. However, at other times, it comes back on and seems to blow cooler air towards my face. What causes the defrost fan to come back on when in Auto mode? is that normal?

Thanks