The old "Gentleman's agreement".
They all kept publishing that number to avoid government intervention.
They all kept pushing over it to be quicker than the other guy.
It's why I'll say it's smart the Gen4 Supra was more suited/targetted to taking on freeway runs, than taking on the GTR directly. Nissan and Yoda rivalry slightly more apart receiving a little less criticism compared to the Holden Vs Ford in the years gone by with the likes of the GTHO etc.
The GTT's Neo actually did produce ~206kW at the flywheel.
The 34GTR's 26 really did not produce 206kW at the flywheel. By the time the 34 came around the 26 was likely up around 230-240 kW, and probably could have been more. The Japanese manufacturer's had an agreement to not sell any cars that made more than 206kW (actuall, their agreement was expressed in the stupid non-metric German ps units which is essentially horsepower, and the numerical value in the agreement was 280ps).
So Evos, Supras, GTRs, 3000GTs, and so on, all had official 206kW power outputs published, and all made variously larger numbers than that. Even the R32 GTR made a decent margin over the official number, all the way back in 1989. As you'd bloody well hope for a TT 2.6l 24v engine running ~12psi of boost.