The idea was for the variant to be set up for free at the start of the game to take account of existing designs/classes rather than give everyone standard 0/0/0/0 level one units.
e.g. Heavy Cruiser 1 >10,000t base level (1922) 0/0/0/0
Admiral Scheer launched 33 -11” guns +2 firepower + 1 armour
Zara launched 31 -8” guns standard 0/0/0/0
Mogami launched 34 - 6.1” guns -2 firepower +1 speed
Refitted - 8” guns +1 firepower + 1 speed
Coding wise should be ok – variant values changes from 0 to +5 to -5 to +5
Need to set up starting (free) variants for each nation for the existing tanks/aircraft/ships if differ from base levels
The negative values allow for older/poorer designs than the base level 1
That particular example is poor because the difference of heavy and light cruisers is based solely on the caliber of their largest guns. All cruisers of the treaties' signatories must not carry a gun exceeding 8" in caliber, and cruisers are further divided into those carrying guns no larger than 6" (later 6.1") and those that do, i.e. CL and CA.
Even after the treaty expired, the principle still holds, and nobody disputes that Yūbari at 2840 long tons and Cleveland at 11744 long tons both belongs to the same designation.
But the Japanese judged the 6.1” design so poor to rebuilt all 4 with 8” guns, the figures used were an example of how a ship can differ over a base 0/0/0/0 design.
As you said would nice to see variations in nations units - British aircraft carriers + armour - deck space etc..
And no, the 6.1" wasn't particularly poor, it's because Japan wanted to build more CA, but their CA tonnage had run out so they build big CL that happens to be able to carry 8" in the same rings. When they wanted true light cruisers, they built the Agano which was much smaller than the convertible Mogami or Tone.
IMHO better examples would be Ise or Mogami's flight deck fitting after turret explosions which made them undergunned at their displacement.