Clarification on createdDateTime and apparent inconsistency for email messages

Question

I've noticed something strange with the createdDateTime value returned for for email messages. When comparing that value (for a newly-arrived message in the Inbox of a mailbox) and the receivedDateTime value, I sometimes see cases where the receivedDateTime is a second after the createdDateTime. I would have expected the receivedDateTime to always be the same or earlier than the createdDateTime, the assumptions being that receivedDateTime is when the message was received via SMTP and createdDateTime is when the message appeared in its current folder.

What am I missing here?