Plastics! No, it's timing.
Simple interest (i.e. no compounding) is just the number of days (or weeks, or months, whatever) times the interest rate for that period times the principal outstanding. So assuming a constant payment amount (of $370), each payment will have a greater percentage of principal, because the interest charged will be on a lesser amount for that period.
Something isn't adding up - pun very much intended - with your post, though. I think it's just "not enough information". With simply interest, it's in your benefit to pay early, because interest is NOT compounded, it's paid in full each period. So if you take out your loan on October 1st, and agree that the payments are $370 starting November 1st, if you pay $185 on October 15th, and $185 on November 1st, you SHOULD be paying less interest not more, because 15 days of that interest calculation is on a lower principal (from the first payment). So without knowing what periods you're talking about it's hard to tell why you're numbers are greater, except in the general sense, TIMING.