I'm not saying it's not a good test, but the inverters can have a more dramatic impact on production numbers than various same-wattage panels... Site annual weather conditions also makes a big difference as well.
In your example of the overcast skies, it's not the panels that are not producing power, but it's the string that is not hitting the minimum voltage required to start the inverter up. In our case, in Southern Ontario, where quite often we get cloudy skies or hazy skies, I've seen some site tests here that an "optimized inverter" can make as much as 75% increase in total annualized power output from a "standard" (i.e. one of the big brand names) inverter.
FWIW.
Victor
www.ecobuilthome.ca