Heat always partially on.

This is my second s10, had a 98 now have a 2003 ZR2 with manual climate control. And to the best of my knowledge, my 98 did the same thing. When the vehicle is at full operating temperature and moving and the climate control is set to off, ( All knobs turned counterclockwise..fan off, temp to cool, and A/c off. ) I notice that a small bit of heated air leaks through the floor vents. Does anyone have this system in there vehicle. Also, same condition, but when the vent is turned to dash with fan blowing, its cool outside air comming from dash vents, but then turn the dial for floor and its warmer air. Just an observation that has puzzled me.Thanks