BATs on turbocharged and intercooled engines are directly related to initial air temperature. Ambient air is almost always cooler than air taken from the engine compartment. The lower the temp of the air charge going into the air filter at the front of the intake tract, the cooler will the the air charge coming out of the intercooler, even though the turbo may add several hundred degrees to the air charge while compressing the air and the intercooler may take most of that back.
There are well established mathmetical formulas that determine intercooler efficiency. Those formulas begin with temperature at the point of earlierst entry of air into the intake system. Work through the formula and the result is that a change by increase in ambient temperature on the front end of the intake tract results in the same number of degrees in rise of the intake charge after it leaves the intercooler.
Sorry for getting us away from the gas mileage issue. How much this difference in initial intake temperature matters can be insignificant or huge depending on operating conditions, which is beyond the scope of this thread about mileage.
