thanks very much again, Herman :)

to my knowledge:
there is no information of the aperture in the body, therefor the Leica M9 and Monochrom (and M8?) measure the light through a small round sensor at the front next to the finder and also TTL and then compares these measurements and calculates the aperture. If you use filters (color or ND), this calculation will be wrong. You find the same effect with high contrast images, with equal contrast the calculated aperture is ok. or very close.
A simple test:
hold your finger over the small sensor at the front and you get different apertures in the EXIF - I hope, this is correct and not boring for anybody :)