Looks like a standard simple interest note. You pay the interest for the days outstanding for the month.
If you take your principal amount for the month (balance after applying latest payment) x the interest rate divided by 365 (or 360) you will get the daily interest. Then figure out how many days between when your last payment and current payment post and multiply by the daily interest. Subtract this from the payment amount and you will get the pricipal paid for the month and the starting point for the next calculation.
It is more like a credit card than a mortgage. If you pay early you will pay less interest, if you pay late you will pay more. For a mortgage loan you will pay the same whether you pay on the 1st or the 15th. In this case the fixed payment is approximate and you will normally end up with a payment or so difference on either side of the term to account for the fluctuations of when you pay over the term.
If you can't make the math work by using the daily interest then you could have a problem. Sometimes there are also mysterious delays
in posting your payment which earns the bank a little more interest. I would try to pay electronically so you know the draft date.