There are two possible answers... There are two types of multi-meters.
Some are "Peak reading" and show you a reading that is as I recall about .707 times the actual voltage Others are True RMS.
Now I'd hope the Inverter generator is True Sine wave or Pure Sine wave out put (There is really only one way to be sure and I do have the test devices)
But if it is not a peak readin meter will read low. but equipment will work (Non electronic that is)...
But I suspect the meter as well. Try a different meter.. In this case cheaper(Like the 5dollar harbor freight job)might just be better.