This does seem like an odd case. Are all of your leads and contacts in SFDC coming down to Marketo? Do you have a custom dedupe filter enabled?
If not, you may want to submit a support ticket for more investigation.
OK, it turns out everything I wrote is a lie. Seriously, I was given premature information, but as it turns out, as a result of a mailing to our distributors to attend a live webinar, several registered with a different email address, and the rest were obviously forwarded the invitation and were not yet in our database. So this is a non issue.
Thanks for your reply, Will.