I find that the NADA stuff often has errors with repect to Airstream dimensions and such. Some widths are incorrect, and it appears that they round their trailer lengths different from Airstream Corp. in many cases. I think what they are describing is a '97 21' Excella that is listed in the "trailer weights" section of the Airstream website here
This appears to me to most likely just be a case of upgrading of the '92 to '96 21' Sovereign
to the Excella trim level for '97, because the weight increased significantly from '96 to '97. I suspect that the floorplan is likely identical to your '93, but perhaps it got the wrap around front windows and other upgrades.
One thing I am uncertain of (since I've never seen one) is did the '96 21' Sovereign
and this '97 21' Excella really get converted to "wide bodies" with the rest of the trailers in '96? I'd guess yes, because the '96 21' Sovereign
is listed as weighing more than the '95 21' model, although it is interesting that later 22' Safari's and International's/CCD's are not "wide bodies".