The difference in the charges could be that 2 of your referral a friend members are no longer with PM or you lost your $2 credit for AutoPay.
Only way to find out is to login to your account and check under Access Rewards. This will list the all the referral members that is currently with PM and the AutoPay credit.
The extra payment will remain as credit in your account for next month's bill.
If you still wish for a refund, you will need to contact a CS_Agent (Moderator) and request for a refund but it may take up to 30 days (time of your next renewal date).