"sent" is likely your default status in the salesforce campaign. My experience, when it rolls to that was that it was a mismatch from the beginning. Can you confirm that "Attended" and "Attended" are in both systems before you update? No extra spaces, or anything else, right?
It even has to be an exact upper/lower case match.
For this reason, I recommend to always create the SFDC campaign from Marketo.