It's possible that your monthly bill is higher than your subscription charge due to additional charges incurred last month. Your subscription plan includes certain allowances, and if you exceed those, then you will be charged for the extra usage. You can check the breakdown of charges on your invoice and statement.
Examples of additional charges include excess mail storage fees, additional mailbox users, and additional mailbox recipients. If you have any questions about the charges on your bill, please don't hesitate to contact our customer support team.