To get the oil to drain as fully as possible from the sump plug, the bottom of the pan would have to be perfectly flat and the car left for a few hours so that oil from all over the engine had time to drain into the sump. How are you to know that the sump pan is not tilted slightly in one direction thus letting the pool stay put? The floor may be flat but the springs would have to be just right to ensure the bottom of the pan was horizontal. What dealership let’s the car sit for a couple of hours any way ?
I suck my oil out from the top, if I feel real keen I drive the driver side wheels onto pieces of 2 x 4 with the engine warm suck out the oil and if I am really keen I let it sit overnight and get more oil out the next day. Not worth the effort really because the amount of old oil eft in the engine after the first drain is really quite negligible, my oil gets changed every 2,000 miles on average or once a year in actual fact. Anyone who drives 7,000 miles before changing the oil is not doing themselves or the next owners any favours.
I put in 8 litres and have no trouble at all, I would say it is slightly above the full mark by a tad but so what?