Junk Contracts

i have a 4 year, 80 000 kilometer buy back contract with BMW for my 220i M Sport 2017. The contract allows me to restructure my buy back deal by reducing the term and kilos or increase the term and kilos. But when i contacted BMW to restructure to a 3 year, 60 000 kilometer i was told my installment will increase by R18 000.00 so i tried restructuring to 4 year 60 000 kilometer and they told me my installment will increase by R15 000.00 which makes no logical sense at all as im bringing in the car at 4 years but with 20 000 kilos less!