A couple of thoughts:
Air or nitrogen (or any gas for that matter) is going to behave the same regardless of the material the container is made out of.
There is a difference in the expansion (or contraction) rate of steel vs aluminum, the aluminum alloy being larger. That means that when the temperature drops, the alloy wheels will shrink more, and - perhaps - there is more leakage. If this is true, then when the temperature goes back up, the alloy wheels will have less pressure.
It's also possible that the measuring device is reacting differently.