Please help me understand Honda Loan Calculator
 

Hello all,
I printed my transaction history and don't understand why the interest amounts (IA) vary over time. For example, the IA in 01/14 was $23 and kept going up to 04/14 at $32. Then going down from 05/14 at $26 until 09/14 when it went up to $30 and then went down in 10/14 at $22.
The monthly payment is fixed at $350 and the interest rate is 1.9% fixed. I pay exact amount in the statement with no extra payment so far.
Honda Customer Service Rep said the IA varies depending upon what date of the month they actually receive my payment. I pointed out to them that I always make payment as soon as I receive their statement in the mail meaning it's always much earlier than the deadline but how come the IA still varies? I also told them that my house mortgage arrival dates vary but the IA still the same for that month, provided I pay before the deadline.
I can monitor my mortgage payment by using the universal loan amortization excel spreadsheet and the difference between my spreadsheet versus the numbers from the bank is 1 cent, even when I make extra payment towards the principal.
But I can't do that with my Honda Loan. Any input is appreciated. Thanks.