Using my GPSMAP 276c as a voltmeter would give me strange voltages as well..
Anything from 11.9V-14V.. (fluctuating between 12.9-13.4 is the most common readout on my unit while cruising at let's say.. 4500 rpm)
I've been using the bike like this for almost a year now, and there is not a problem with it..
I guess it's only the GPS's voltmeter, that's not that accurate ( I guess it under-measures the correct voltage by a 0.5V minimum).
I wouldn't be scared if the volts stay between ~12.0-~14.5V (when engine is running at any rpm)
just my $0.02:)
2005.5 950 Adventure (with 265mm travel suspension