Presumably you are using either a silver-oxide cell in an adapter (in which case the bottom of the MR4 probably doesn't close too well) or a zinc-air cell of some sort ? If the local photo-shop sold you an alkaline battery to replace the original mercury one then the readings will be both off, and inconsistent.
I use an MR4 on my M3 too, with zinc-air cell in a washer, and it is consistently less than 1/2 a stop different to my calibrated meter so it's also possible that a previous owner has 'adjusted' your example. In my experience these meters work as well as any small incident meter, and are very convenient because the shutter-speed is linked when it's mounted on the camera.