I have found Mercedes to be very specific regarding measuring oil levels (and other fluids for that matter).
It is very important to check the oil level are a very level surface. A car parked on a slight incline can change the actual level measured on the dip stick.
Also, it has been reported that some engines when filled to the upper mark on the dip stick tend to use oil quickly down to the mid mark then not use much more. I have experienced this and I don't know why.
This is what I do: I check the oil when the engine is cold, say after sitting over night. Then I am assured of exactly what the level is, repeatedly.
What weight oil are you using?
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)