It looks like you have external users, and are using Web Email Protection. Note that it will 'process' those emails, but the actual message is not 'encrypted', it is stored on the server while a link is sent via email letting the external party know they have mail.
Encrypted should only refer to messages where a valid key was found, and the message was encrypted at the server before passing out of the network.
If messages are processed (encrypted) by Symantec Encryption Desktop, they would effectively be a 'normal' mail as far as the server is concerned, as the server itself is not processing it.
The same goes for decryption. If messages are decrypted on the desktop by the Outlook plugin, the server did not decrypt them, so it will show no stats for decryption. Those are only for mail which was decrypted at the management server level.