Yes, you have Orphan fields in SFDC.
Let's say you mapped this in SFDC
- Lead Source on Lead > Lead Source on Contact
- This appears as "lead Source" in Marketo.
Let's say you did NOT map these in SFDC, you could end up with
- Lead Source (L)
- Lead Source (C)
- Lead Source (A)
and when you use each one, you will get totally different results because they are different REcord Types. So you would need to use Lead Source (L) OR Lead Source (C) to get both leads and Contacts.
Same issue if you map, or don't, to Account object instead.
But since your fields is mapped to the Account, Marketo cannot update Account fields, only Contacts. I would suggest creating or mapping to a Contact field instead/addition to. Your SFDC Admin can help more on this.
if you wanted to fully correct this, it may be possible, but it's a pain. I'd just start to map to the Contact field. Then, in SFDC, use logic to handle the Account field if you need it. The Account field will only be updated on the initial creation anyway.