NetZero may not have been able to collect your monthly payment in the previous month/s (Click here for reasons as to why this could have happened), and so you were charged the balance due as well as the current month's charges on your last billing date.

Or,

You may have recently upgraded your service to NetZero Satellite SpeedSaver or added another NetZero service, such as MegaMail Plus or our Family Pack. When you upgraded, you may have incurred pro-rated charges, which carried forward to your next billing date. Hence, you were charged an amount that is different from the advertised rate.