I was told by Marketo that the activity was associated to the contacted in Marketo with the lowest numerical value in the Marketo Contact ID field.
So if you had Susie Smith and Susie Smith had a Marketo Contact ID in the People Database of 1 and she also had an ID of 200 - the activity would sync to the contact with the ID 1.
Ok thanks! That's a different answer than the one I got from our contact from Marketo, but she had to check, so you're probably right. We understood that the lead will be linked to the account with the most recent activity, which also sounds probable, but again, she wasn't sure.