When the engine is running, it sucks up the oil from the reservoir and puts it in all the galleries, ie the oil's all in the right places to lubricate the engine.
When you shut the engine down, the oil starts returning to the reservoir by gravity. So there's a gentle drip-feed back into the reservoir, which means that the apparent oil quantity measured at the reservoir is increasing.
So the problem is WHEN should you measure the oil quantity - when just before the oil starts returning to the reservoir, or after that process has completed? Well, it can take a while, so the general convention is to read the oil quantity, and to add oil as necessary, before the oil has gone back to the reservoir.
(Don't know why the APU has anything to do with this.)