The air coming out of the case fan passed through the case and picked up
heat on it's way through from the components inside.
The air coming out of the PSU fan didn't just pass through the inside of
the case, but it also picked up some heat on the way through the PSU.
The PSU fan is intended to draw air through the PSU, acting as both
cooling for the PSU and to help exhaust the hot air from the case.
As the air coming through the PSU fan has been heated up by a second
radiating source, I would expect it to be a bit warmer.