interest rates

RCS interest rate is half my installment and i tried calling their call center twice the lady who assisted me for the first time she told me the system calculates the interest automatically and she cannot explain why half of the installment diapered and why is it half of my installment and the gentleman who assisted me he kept telling me about overselling which he couldn't even explain the word itself and tried to break down how their system concluded half of my installment disappeared,