Poor service

i am quiet confuse on how your business works, so i worked in the financial and retail industry for years now and had accounts for years , i even know when the prime rate changes your bond installment drops , my concern i have is that i need to understand how come my installment is R625.00 on a balance of R3500 ,all other retailers drops your installment as the balance decrease the installment goes up as the balance increase... the store consultant told my that my problem if other retailers does this