I run into that issue with a lot of different cars. I think that somehow the fluid gets splashed or sucked up the tube. You have to remove the dipstick, wipe it off, then reinsert before taking it out to read the proper level. Even then, you can run into problems with the dipstick rubbing against the wet walls on the tube giving false readings.

__________________
95 E320 Cabriolet, 169K