The Created By field pulls up a List/Record for all entities stored on the account. However, Filters found in the footer section of any Saved Search is limited to show only 1000 records. Therefore, if there are more than 1000 entities stored on the account (e.g. Employees, Customers, Vendors, Partners, etc.) then the list is truncated and will be aggregated to the following: All, Mine, or Mine Team. The ability to increase the limit for entity footer filters is currently filed as Enhancement# 170846. The alternate solution is to manually add the filter in the Criteria instead.