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Motorbike v 4WD Greenslip Cost

Please explain why a CTP insurance is twice as expensive for my Motorcycle than my Landcruiser 4x4. If I understand this scheme correctly, I am far more likely to cause serious injury or death as a ""at fault"" driver/rider , to another road user, in my 2.7 ton 4x4 than I ever will, riding my Motorcycle, yet I pay twice as much??