The most likely reason is that there are two separate subscriptions or charges rather than one yearly plan converting into a monthly charge.
Common causes supported by the billing guidance:
- A second Microsoft 365 subscription is active
A normal Microsoft 365 Personal subscription is usually billed either monthly or annually, not both. If a yearly charge happened in July or August and there is now a $9.99 monthly charge, that usually means there is another subscription on the same or a different Microsoft account.
- The charge is on a different Microsoft account
Microsoft recommends checking Services & subscriptions and Order history with each personal Microsoft account that might have been used.
- Another account using the same payment method
If only one charge appears in one account’s order history, Microsoft notes that multiple accounts in a household can use the same card.
- Recurring billing is enabled on another subscription
Unexpected Microsoft charges are often caused by a subscription with recurring billing turned on.
What to check:
- Go to Services & subscriptions and look for:
- a yearly Microsoft 365 subscription
- a separate Microsoft 365 Personal subscription at $9.99/month
- whether recurring billing is enabled
- Go to Order history and identify both charges:
- the annual charge from July/August
- the recent $9.99 monthly charge
- If the $9.99 charge does not appear in that account, go to Manage your payments and select Investigate to check whether another Microsoft account is using the same card.
- If both subscriptions are active and only the monthly one should remain, open Manage on the unwanted subscription and cancel it or turn off recurring billing to stop future charges.
If the annual and monthly charges are both Microsoft 365 charges, that is usually evidence of two subscriptions, two accounts, or a purchase through a different billing channel.