Yes a big shock for us too when we purchased a 2018 California Beach from Exeter commercial vehicle dealer Carrs.
Totally outrageous profiteering by the Government / DVLA.
I phoned DVLA asking them to explain themselves and ask why my California would exert more wear on the roads to say another van, panel or otherwise, of similar weight, naively assuming that road tax goes towards road repair.
As stated here, it's all about the initial value of the vehicle and it would seem that from DVLA's viewpoint, you are rich so you can afford to be further penalized, so the policy is to rip us off.