Have you checked in Admin > Field Management (Fields Tab) > find the value "Source" (under folder "Lead Info") > where it says "Block Updates From" > if this shows updates are blocked by form fillout then a Marketo form isn't going to overwrite whatever existing value was in the Lead Source field on the Lead you tested on form submit.
Blocking Lead Source is common practice to ensure only the first "origin" value is ever captured for a Lead.
Try submitting a Lead with a new, unique email address as a test via each form > are the Source values unique?
If the above doesn't resolve things, check your test record's Activity Log in Marketo and click on the ID number of the form fillout activity to see what data was submitted via the form. Was it captured correctly? Check if there were any following Data Value Change activities logged to Lead Source.
If none of the above leads you anywhere, its possible your field in Marketo isn't mapped to the Source field that you're looking at in SFDC (check Admin > field mapping), or that there is a trigger in SFDC/Marketo that is updating the Source value on creation of the Lead regardless of what was captured via the form.
Hope that helps!