Salesforce Administrator shown in 'Last Modified By' field

This 'System Administrator' periodically runs a background process that cleans up metadata associated with deleted custom fields. This process will affect the Last Modified Date and Last Modified By fields on page layouts, record types, and custom objects.

There is process that cleans up metadata from the database that is invalid. The particular part of this "Physical Delete" process that caused this update is the part that was cleaning up expired soft deleted custom fields. A custom field that is deleted by the administrator is not deleted immediately, but is marked as "soft deleted" and stays in the system for at least 30 days, and then is "hard delete" when this process runs.

In an org where a custom field had been "soft deleted" for more than 30 days. In the process of "hard deleting" the custom field, the Salesforce administrator removed the reference to that field from the Layout/record type. Since the layout/record type had been modified, we mark this by saying it was modified by "Salesforce Administrator".