No you're wrong to a point, it's just not that way in all cases.

Let's say you were awarded ATL900 CA as primary, you were originally DTW900 CA. You've created a secondary vacancy in DTW 900 by bidding to ATL. With that vacancy an FO was awarded the last spot in DTW 900 CA (after which alternates would fall below him). If you withdrawal your bid, you go back to DTW 900 CA and that FO gets bumped out of his upgrade because you deleted the secondary vacancy by withdrawing. If the FO withdrawals (and you stay in ATL) then an alternate would get used to fill the spot the FO left open.