It's not necessarily how he measured, but what he measured
The measurements of the KEF900 were actually at tweeter's height and at 50" mic distance. They pretty much measure all of their speakers the same way. But this is what's interesting to look at:
Look at the green trace, the woofer(s) response. Major break-ups between 5500Hz-9500Hz. It even shows up in the red trace, which is what Stereophile usually only posts. By looking at the medium sized wrinkles on the red graph, it's hard to judge the true performance, unless we can see the individual driver's roll-offs, or distortion graphs (which Stereophile doesn't publish for some reason). So yes, the overall SPL looks more, or less acceptable. BUT! If the region between 5500Hz and 9500Hz is excited by music, the distortion would literally be through the roof! And right smack in the midrange, no less.
It's actually a very poorly designed speaker.
P.S. NHT has also been reported to sound rather harsh in it's upper midrange and I suspect the same lack of care in the crossover holds true for the overrated Classic Three. Take a look at these measurements:
SoundStage! Measurements - NHT Classic Three Loudspeakers (3/2007)
Notice it has the same break-up visible in the overall SPL graph? Now if you look at the distortion graph, this same region (6000Hz-8000Hz) all of a sudden spikes.