Yeah, it's the heat in the engine and oil; the engine parts expand and the tolerances get a bit wider, and the oil thins out a bit.
My '91 103 engine will routinely drop below 1bar (15 PSI) at 600 RPM in Drive on a hot day. it's done this since it was new. The experts say that at idle, any pressure over 4 PSI is OK; and the general rule is 10 PSI for each 1000 RPM.

DG