Temperatures: outside air versus vent air

[Jack500]

On my 500 SE, 126 chassis, I notice that when the push button is set on EC and temp on MIN for outside vent air, the temperature coming thru the vent is slightly warmer than the actual outside air temp. This occurs also at nite or cloudy days and so it is not due to radiant heat buildup. I have measured the temp. differences at about 8 to 10 deg. F. (70 deg outside & 78 inside) Is this normal? I do not detect any heat coming through the heat exchanger but I cannot be absolutely sure. When it gets real hot, I don't want my AC fighting the heater.