I have not checked this on my car but I expect it to be normal.

Wiring has what is called voltage drop, if you put a high load on the end of a wire, the wires resistance lowers the voltage at the other end. Take the load away and voltage will pop back up again. The climate unit is seeing voltage after a long run and probably through the ignition switch.

Think of it as a garden hose. With the house valve on and nozzle shut, pressure will rise and swell the hose. Open the nozzle and the hose will shrink a bit due to running pressure being lower.

Alternators and really good lab power supplies have remote sensing wires that carry no current and are there for voltage sensing. This keeps voltage consistent on the other end.