the A/C settings its best to set the air to come out of the upper vents and direct it upwards so it's natural tendency to settle to a lower level helps keep the temperature consistent in the entire cabin. Conversely when using heat, the lower vents are used so it can rise and keep the cabin temps even. When using floor vents, it is assumed that you are using heat, and that the windows may need defrosting if it is cold enough that you need to keep your feet warm. I see you are from a southern state, so this may not be so obvious.
