I have a few multimeters, none of them Fluke class. Two from CanTire I bought 10-15 years ago have been steady and agree. Some cheapies and small that I take on the bike for emergencies, but they aren't far off the good meters at 12V range. I bought a $60 one from China since I needed one with RMS, and it is very good, my favourite now. All my decent ones agree within a tenth or two.
I also install voltmeters on every bike I've had (at least the last 8), to monitor power usage/charging. Not expensive, and cheapie ones of those can vary, yet most surprisingly accurate. A Datel or the knockoff would be the good reference.
Just sayin, check batteries and test leads, perhaps use same test leads on each to see if consistent.... all should be close.
As for the bike's charging question, I have not had to have any dealings with an ST13 alternator.... my observation is that voltage regulators should not allow more than 14.5V, almost all hover at 14-14.2 while cruising (say 3000 rpm or above). Condition of battery can affect this, maybe want to load test your battery first (free service most places unless you have a tester). What you watch for on your bike voltmeter is excessively high voltage (~15) or excessively low voltage, which could indicate a regulator issue, assuming the battery is good.
Yuasa says fully charged battery at rest is 12.9V for an AGM, 12.5 is 50%. This is after fully charged and surface charge dissipated. A typical Battery Tender maintains a battery at 13-13.2V after full charge is reached.
Hope that helps a bit.... older Yuasa chart attached.
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