This is working as expected. The Parent Company Name field gets its value set from a different process and not via the SFDC Synch.
SFDC and Marketo Engage both have a Parent Account ID field in the Account Object (Company Object in Marketo). If there is a parent account ID in that field, then the value of that field is automatically populated via a reference to the other Account Record. Since Company in Marketo is mapped to Name (SFDC Account) whenever that value is updated it will be reflected in Marketo.
Example: Account ID 1 has a Parent Account ID of 2. They both have records in the same Account table. If the Name is changed of the Parent Account it is automatically reflected in any child accounts as the Parent Account Name is just a reference.