How loans are calculated

Loan calculations using a declining balance calculations are known to be very exact, however there can still be variations for a number of reasons:

Rounding Errors
A loan calculation uses one formula to determine the payment needed to pay off the loan over the loan period. It does not take into account how a bank may account for rounding during the course of the loan.
Even in the most rare situation, where each payment was rounded against the borrower’s favor by 50 cents, the total error even over a 30 year loan would be minimal.

Lending Rules
Different lenders and regions have different rules when it comes to lending. There may be additional fees (either at the beginning, end, or over the course of the loan) that may incur additional costs.
Unless stated or a field for those expenses is included, a loan calculator only includes the loan portion of the loan, and no other costs.

Calendar Quirks
How the lending institution accounts for calendar days and holidays can have a small impact on a loan calculator’s results. Different policies would affect shorter compounding periods (such as daily or weekly payments or compounding periods) more than they would longer ones (such as annual or semi-annual payments or compounding periods).