To help you understand it, the system is designed to not have the oil sitting in the engine,(hence- "dry sump") this way it can be a shorter/more compact motor, among other things. There still will be a small amount in there, maybe 1/2 quart, but otherwise the pump is always sucking the oil out and pumping it back into the tank.

There is a check valve inline from the tank, that is supposed to hold the oil from gravity flowing down into the motor when it isn't running so when you pull the dipstick all the oil that isn't supposed to be in the motor is in the tank, where you can read the amount. If your check valve is leaking, every time you shut the motor off, the oil in the tank will slowly flow down and fill the crankcase(like sand in an hourglass), it usually doesn't just dump out, but flow slowly like a seeping seal, may take a day to empty the tank, may take 4 weeks, depends how bad the check valve is leaking.

Anyways, because of that design, when you check your oil level, you need to start the motor so the pump can suck any oil that may have leaked down there, out of the crankcase(it pumps more out than what gets pumped into the motor) and put it back into the tank where it can be read. That is why a person is supposed to run the motor, then shut it off, wait a minute or 2 (lets the oil settle in the tank), then pull the dipstick to read it.
If you don't first run the motor,the oil that may have seeped past the check valve won't be in the tank and a person will add oil to get it up to the level but now it will be overfilled.
Hope that helps, it's my short version of how it works