i have joined that group since i've seen you mention it, but haven't really read much there yet.
i have the solarmeter 6.2 which just displays uW, vs the 6.5 that displays UVI, but UV index is based off uW/cm2 so it's just different ways of expressing the same measurement. i know the two meters have slightly different response curves, but don't know that one is necessarily more "accurate" than another. they're just inaccurate in slightly different ranges. short of using scientific instruments, we just have to accept those inaccuracies and do our best. there are certainly differences between natural light and artificial light throughout the UV spectrum, but these two meters are really designed primarily for measuring one small area of that spectrum, the UVB range that is useful for D3 production in reptiles. neither meter is going to tell you if you have dangerous levels of UVC or insufficient UVA (though there are other meters for that). so you can't really come to any conclusions about the spectral response of artificial vs natural light based on either of those two meters alone outside of that narrow UVB range.
i do wonder about the distance recommendations on some bulbs. for example the mega ray 100W MVB recommends a minimum safe distance of 12". well, at 12" i get around 120-150 uW/cm2, well below natural sunlight values; i don't get near 250 uW until i'm around 6" away. so i wonder if those recommendations are based on some other factor. perhaps UVC exposure is too high at those distances and causes eye problems. or maybe there's enough variability that some bulbs will put out much more UVB at that distance. mine has been in service about 5 months, so should still have plenty of useful life left.