Why Does my Floor Position Take Long to Turn On?

I noticed this yesterday. If i start my car, then switch the knob to FLOOR, and leave my fan speed on AUTO, it takes like 1 minute to come on-but if i manually turn my fan speed knob to med-to high, it comes on right away. This only happens with that FLOOR position-which i like most for heating the car. I keep my temp at 71deg..

thats weird. one of those crazy unexplained mysteries in life. heres another example of one...Why is my t bird slow as s***? who knows? it was fast in hawaii before I had it shipped to ohio.