If you have multiple internet users in your household using the same account, internet activity for all users will be aggregated at the account level. So it may be that other users in your household may account for the usage.
If you use a WiFi router in conjunction with your internet service, that WiFi connection may not be properly secured and other users outside your household could be using the connection, adding to your data transfer usage. This can also present a serious security risk to any personal data on your computer(s).
If you subscribe to our Wireless Home Networking service and are unsure if your WiFi connection is secure, you can contact our 24/7 technical supportgroup to verify. If you are using your own router and are unsure if your WiFi connection is secure, follow the manufacturer’s instructions to verify.
You may have a virus or malware on your computer that accounts for the additional data transfer usage. Properly installed and regularly updated anti-virus software should protect you from this possibility, in most cases.