Why compare official treaty tonnages?

Is the comparison using the official treaty tonnage even when navies understated displacement?

Yes; official declarations put every navy on the same documentary basis, even if all of the figures contain distortions.

The comparison uses the tonnages each navy officially supplied for the Washington treaty. Those figures may not represent the ships' true displacement, and the understatement was not confined to one fleet. Using the same documentary category nevertheless keeps the comparison internally consistent. The method compares the navies on a common level of declared data rather than mixing official figures with later corrected measurements.