I'm having the exact same problem. Here's what I think is happening:
A user converts a lead to a contact in SFDC
During the brief moment the contact record does not have values in the name, address, title, phone and a couple of custom fields, a sync to Marketo occurs (SFDC does have the values when the conversion process is complete
Once the conversion is complete there is no subsequent event in SFDC to cause a sync to Marketo
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