Were you loading it when you got those output readings? A stable system will put out a couple amps more than what is necessary and no more. If you turned on 50 amps worth of users and only got 55amps, it would be what I would expect.
To get full output from the alternator it must be loaded externally (as the cars loads will usually be less than rated output - except for the cars made in the 80's)
Bosch Master, ASE Master, L1
33 years MB technician