you'll find typical alternators put out anything up to 15V when running fast
and under light load - put the headlights and heater blower and wiper motor
on and measure it... and heated rear screen if available.

I don't think the voltage regulation on automotive generators is all that
precise.

I once, on one of my cars, fired it up, moved it about 20 feet across the
yard and left it running.  As it was damp sort of weather at the time, the
HRW was already on.  After a minute or two the car stopped of its own
accord, which I thought little about, then after about 10 minutes I went
back to it and found that the HRW (plus other ignition style loads) had
flattened the battery - but just leaving the ignition on doesn't normally do
that in 10 minutes, the HRW takes a fair few amps.