IIRC, the AC can only remove so much heat from the air going across the coils. It seems that the maximum amount varies based on the refrigerant, but let's say it is 30 degrees.
When it is 80 degrees outside, that means the air coming out of your vents would be about 50. If the ambient temp is 105, the vent air will be about 75. Stick a thermometer in your vent and see what it says...