With no load and revved engine, 14V is about right.
Alternator output varies with revs and only reaches the 75A rated output around 3000rpm.
At idle the output is about half the 75A rated output.
With the 56A draw at idle part of the load was being supplied from the battery.
It is likely that your alternator is OK. The battery may be faulty or you may just be drawing too much from the system.
Check that the alternator drive belt is not slipping under load!