While it might look that way, the interest is recalculated at month end on most mortgages in the US. The amortization schedule is set at the beginning based on the MINIMUM payment expected to align with the term of the note. This allows them to charge correct interest on the note regardless of payment behavior (over or under paying changes interest owed). This is why overpaying is advisable, it reduces the total interest you'll pay over the life of the loan.