Auto Loan Question

[kanderna]

Hi all -

Hopefully someone here can help identify what's going on with my auto loan. It has been going on since opened, but just find it a bit odd. What I show as my current balance vs. what they show is off by about $600 in their favor, so I thought I better take a closer look.

- Opened in 4/08, and I have a fixed 4.99% rate.
- Monthly payment is 456.53, but I always pay $500 and apply the additional to the principal

Here are the amounts from my last five statements:
Principal + Interest (Extra Principal is always $43.47)
387.80 + 68.73 (April)
354.28 + 102.25 (May)
382.73 + 73.80 (June)
366.30 + 90.23 (July)
385.92 + 70.61 (August)

I've seen some explain it by saying different number of days in the month, but there are $30 swings in there. Hopefully someone can lend assistance.

Thanks!