My understanding is that this is the Default value for leads created in SFDC, so Marketo will know they originated in SFDC.
Is Marketo's data showing something incorrect for leads you know were created first in SFDC?
For example, based on one contact I have in Marketo and SF:
Marketo > Original Source Type: Web Page Visit
Salesforce > Original Source Type: salesforce.com
This is the same contact, with the same field, but showing two different values in each side. I want to match up what's in Marketo (which is the accurate data) with what's in Salesforce.