Custom fields in related records

I added the same custom picklist field in both Contact Field Setup   and  Account Contact Field Setup. Both have the same system HTML field tag of "ZPick_0".  I added the field(s) to both my Contact and my Account Contact layout.       From the related info Contact tab at the bottom of the Account screen, I have 2 options:  Edit brings me to the Account Contact layout;    selecting the hyperlink associated with the Last Name brings me to the Contact Layout.  If I update any of the standard fields in either layout, the values flow to the other.  But, my custom field only updates the Contact or the Account Contact.