When a Lead/Contact is deleted from Salesforce it is not automatically deleted from Marketo. If you want to delete these left over Leads/Contacts do the following:
1. Create a campaign using the SFDC Is Deleted filter set to True for the Smart List.
2. For the Flow step choose the Delete Lead action and set the Delete Lead from SFDC to True. This will ensure that the Lead/Contact will be deleted from both Salesforce and Marketo.