The person account type can be tricky. It is a conglomeration object so to speak for Marketo. Its a hybrid Account Object and Contact Object. In Marketo it looks like a regular Contact with an associated account (itself) . By default Marketo does not enable the ability to write back values to the Account Object. This is because in a typical SFDC integration the Account has a one to many relationship. So if one contact updates an account field that would sync back and then update ALL other contacts under the same account. It can get very messy and confusing.
In the case of the person Account it makes sense, and you can make the request to support to enable this for the Person Account. You would just need to make sure the profile your sync user is utilizing in SFDC has the appropriate field and object accesses to write the new Marketo values on the Account fields.
Hope that helps.