Assigning credit for a Lead can only be determined by someone with intimate knowledge of a company's marketing strategies. If someone fills out a form, Marketo doesn't know if they were brought to that form by an email blast, TV commercial, talked to a salesperson, or any number of other reasons. If they came in through a List Import, they could have attended a Tradeshow and signed up there, they could have been exported by a CRM and are being imported, or any number of other ways. There are many discussions on best practices for handling Lead Source attribution in the Community. If you need assistance setting up Smart Campaign to update the Lead Source based on your specific business criteria, please contact Marketo Support.