On a simple interest contract, interest accrues on a daily basis based on the remaining unpaid principal. As each monthly installment is paid, the payments will be applied first to any interest that has accrued on the unpaid principal balance and then to the principal balance remaining on the account and/or other fees as applicable.
If payments are received every month on the due date as detailed on the contract, the final payment due at maturity will be the same as what is listed as the final payment on the contract.
If payments are not received by the scheduled due date, the final payment can be greater than or less than the scheduled payment amount that is listed on your contract.
For example, if payments are paid prior to the scheduled due date, less interest accrues and the principal balance will reduce at a faster rate.
Conversely, if payments are paid after the scheduled due date, more interest accrues and the principal balance will reduce at a slower rate because the interest that has accrued is always paid first.