Strangely, my intital comment appeared to be posted by David P...And I don't know any David P.
So here is my take on this issue:
From what I understand your Contact field did not exist in your initial fields sync. Upon ititial setup, It would have mapped this field too, but if it's been added afterwards, it's no longer automatic. You will need to ask Marketo support to do the 2 to 1 mapping from the backend.