Nice work djozz, once again!
I suppose we are assuming the Class A/B Mobilux A meter is pretty much the gold standard. It's revealing and somewhat disconcerting to see the variations, and also disconcerting to hear that maybe the NIST calibrated Extech meter in the $100-$500 price range so praised about in our community, doesn't even meet a class C rating of errors up to 20%. This could explain why some have seen very little difference in the LX1330B and an Extech.
By comparing the meters side by side as you did, eliminates a lot of variables - temp, pressure, variations in battery effects over multiple trials, etc., so the results seem very real and should be reproducible. Of course I would like to see more common lights/LED's in the test, but obviously you have limitations.
Don't feel the data, testing and info you provided here is under appreciated from the lack of much response. It's going deeper than many want to go. I subscribed to this thread from the first day but held off posting.
Also, all those discussions going on about universal standardizing our meter calibrations seems out the window now, or radically changes it - we'd have to take tint/emitters into account in calibrating our readings, and that's not always well known.
Personally, I was looking at a data logging model of an Extech to buy, but now, I'm not sure it's worth it, accept for the better features, the precision may not be much improvement at all.