Variation in approach to VED tax.

I have a VW Touran 1.9 TDI, and our son has a VW Passat 2.0 TDI. My Touran tax rose gain this year and is now £190, whereas his Passat tax has remained at £30 year on year. There is little difference between the two VED rates, and I did once calculate that if his was correct at £30, then mine should be £57 on a pro rata basis. How can it be right that one VED rate should be subject to increases and another one not? Even though he gains, he still agrees that it is morally wrong.