Could be a few reasons - you could have an extra (auto) payment applied, could have a payment belonging to another finance arrangement applied incorrectly, could be the original interest schedule was different (changed, refinanced), sometimes the first payment has no interest calculated, and you might have factored interest into that first payment.
If none of those apply, and there are no more payments to make on the finance, then to rectify do a journal entry, (Dr or CR Loan Interest, Dr or Cr Unexpired interest) for example - or your accountant will be left to sort this out at year end
Regards