I am using APRWIN 6.2 to calculate the APR for a short-term high-interest installment loan.The payment plan spans up to 20 payments which are 14 days apart with different payment amounts. Although an ideal repayment plan is for about 14*20 = 280 days, I see that the APR changes tremendously based on just the difference between origination date and the first payment due date.
First payment period APR
I do not understand why the APR rises by 140% just because the first payment due date changes from 14 to 7 days from the origination date. Is this correct? Is there any documentation which explains the formula being used to understand why the APR is highly weighted on the first payment due date instead of the total repayment period?