If you look at the panel, just at the bottom of the specs, under the fuse rating- you will see the ratings are for STC (standard test conditions) ie 25C cell temp inside the panel, at 1000W/metre squared, and an 'atmospheric mass' of 1.5 (the thickness of the earths atmosphere from sea level to 'space' vertically is 1 A.M)
Anything else other than those conditions the panels can be making less (normally) but sometimes more...
NMOT is 'more realistic' in 'some' conditions' but can be just as out as STC in other places and conditions...
It basically depends where you are on the planet, time of day, season, altitude above sea level, air and panel temps, and how much light is falling on the panels as to exactly what you will be at...
(which is why I always advise people not to 'crowd' the limits of their charge controller- ESPECIALLY the PVmax rating- in certain weather conditions you can see the Voc of the panel rise to 10-15% over the STC rated voltage- so I always say leave a 20% 'buffer' on the PVmax rating ie the combined Voc's no more than 80% max of the PVmax rating- eg on a 150v PVmax rated controller, the combined Voc's no more than 120v (80% of 150v)