I have noticed a variation when revving the engine,but only slight.I assume that the alternator is charging OK as I normally have no trouble starting, or any other electrical problems,except the rear screen heater not working but who uses it anyway??!! I tend to lean towards a miscalibration.I noticed yesterday that after starting the gauge climbed to 13v and stayed there for half an hour before dropping back to 12.5v,I assume this is after it has replaced the drain put on by starting.
Cheers.(You wouldnt believe that I am an electrician,industrial and domestic,never could understand the peculiarities of car elactrics!)