A B727 FLT CREW INADVERTENTLY OVERSHOOT THEIR ASSIGNED ALT.

Narrative

WE WERE ON DSCNT INTO ATL. WE WERE BEING VECTORED BY ATL APCH CTL. THE AUTOPLT WAS DISCONNECTED DUE TO NUMEROUS ALT; HEADING; AND AIRSPD CHANGES. THE APCH CHKLIST WAS BEING RUN AS WE WERE CLRED FROM 10000 FT TO 9000 FT WITH A HEADING CHANGE AND SLOW DOWN. 9000 FT WAS SET IN THE ALT WINDOW. PASSING THROUGH 8500 FT AS THE FO WAS FLYING; I SAID 'I MISSED YOUR 1000 FT CALL; PASSING 8500 FT FOR 8000 FT.' THE FO ACKNOWLEDGED. WE BOTH THOUGHT 8000 FT WAS OUR CLRNC. REACHING 8000 FT; WE REALIZED THAT 9000 FT WAS SET IN THE ALT WINDOW. I QUERIED APCH CTLR ABOUT OUR CLRED ALT. HE REPLIED; 'THE CLRNC WAS 9000 FT; BUT YOU ARE FINE WHERE YOU ARE.' I ACKNOWLEDGED 'ROGER 8000 FT.' THE REMAINDER OF THE DSCNT AND APCH WAS UNEVENTFUL. THERE WERE NO OTHER ACFT INVOLVED. I BELIEVE THIS HAPPENED DUE TO THE HIGH WORKLOAD AT THAT TIME WITH THE AUTOPLT OFF. DUE TO THE NATURE OF THE B727 AUTOPLT; MULTIPLE PHASE OF FLT CHANGES ARE DIFFICULT. I DON'T KNOW WHY THERE WAS CONFUSION ABOUT WHICH ALT WE WERE CLRED TO. WE HAD A NEW SO RECEIVING IOE TRAINING AND THAT MAY HAVE ADDED TO THE CONFUSION.