whats causing my decreasing boost curve

[Skym]

It's more when take into account clutch slip, around 25% for R34GTR (that was a guess based on calculations) and 35% for R33GTR, R32GTR (that's what he has tested on engine, chassis dyno) according to a head engine manager / engineer (built the Gibson Group-A R32GTR engines) who has tested engines on a engine dyno, on chassis dyno (installed in car).