Why does Sophos Anti-Virus identify an Excel file attachment as encrypted?

Microsoft Excel provides several layers of security and protection to control who can access and change Excel data. One of these layers of protection is password protection, which allows only authorized users to view or modify data. Not having this form of protection might lead you to believe that the file is not encrypted. However, the spreadsheet may still be encrypted by other protection, such as workbook protection.

If the Sophos Anti-Virus engine identifies a message that has a spreadsheet attached as encrypted, open the spreadsheet and check to see if protection is used on the spreadsheet. Excel protection is typically configured from the Protection submenu under the Tools menu. Please consult the Microsoft Excel help menu for more information on Excel spreadsheet protection and encryption.

This might also happen with other formats like PDF where only parts of the document can be write protected. Please look very closly at the document and the setting with produced those protected/encrypted document.