I have USAA and one payment for every cycle I get the same thing. Here's what happens, keep in mind I'm using round and random numbers to make the math easier.
They have insurance cycles of 6 months, so let's say your insurance for every 6 months is $650. Now, they may decide to only charge you a minimum payment of $100 a month, this means you still have $50 at the end of the 6 months to pay. Now, don't ask me why they don't just divide the $50 between the 6 months, I don't get it either. But you have a certain amount you have to pay every 6 months, with the option of paying a fraction of that amount every month. At the end you make that last payment, and whatever is left over, then your insurance renews for another 6 months and it starts over.