It has been suggested by others that you must remove the o-ring from the cap portion of the dipstick in order to get a good reading. This because with the o-ring in place, compressed air is pushing the oil level within the dipstick tube downward.
I do remove the o-ring when measuring oil level, and I believe that I get a better measurement. However, I still get different readings on either side of the stick. Different by about 3/16 inch.
To my knowledge, no one has satisfactorily explained why that is so.