TED,
You must post positions on an AE if you intend to replace attrition.

What you are thinking of is backfill. Backfill happens when someone "bids" out of a given position for another position and that vacated position is subsequently filled in the same AE process. This is when you'd see positions awarded that were not on the AE. It was due to vacancies created by the posted AE, not attrition/retirements.