An expired message is one the carrier could not deliver before its validity period ended, usually because the phone stayed off or out of coverage; it was attempted and is billed like a sent message.
Expiry is a final status distinct from failure: nothing was wrong with the number or the content, the handset simply never became reachable in time. It is common on old lists and in regions with intermittent coverage, and it shrinks when validity is longer for non-urgent content.
Because the message was accepted and attempted by the carrier, it is billed; the carrier charged for the attempt too.
At SMSMeteor: expired messages appear with their own status and are counted separately from failures in the report; the message.expired event is available on webhooks.