B757 ALTDEV ARRIVING DEN.

Narrative

WE WERE LEVEL AT 17000 FT MSL; 250 KTS ON LANDR 2 ARR TO DEN. APCH CLRED US TO 7000 FT MSL. FO CONFIRMED '7000 FT' AND SET IT IN MCP ALT. I (PNF) CONFIRMED READBACK AND SETTING. WHILE DSNDING THROUGH APPROX 12400 FT; APCH ASKED TO WHAT ALT WE WERE DSNDING. FO STATED '7000 FT.' HE INSTRUCTED TO LEVEL AT 11000 FT; THAT WE HAD BEEN CLRED TO 13000 FT. WE DID SO; CONTINUING TO A NORMAL APCH AND LNDG. THERE WAS NO TFC CONFLICT. SUBSEQUENT DISCUSSION: 1) BTWN PLTS -- BOTH RECALL 7000 FT AND CONFIRMING IT ON READBACK. 2) BTWN ATC AND ME -- I WAS TOLD AGAIN (BY TELEPHONE) CLRED TO 13000 FT; SAID HE CONFIRMED IT ON THE TAPE. SPECULATION (ATC): AT TIME OF ALT CHANGE; ATC TOLD US TO 'EXPECT' RWY 7. HE BELIEVES WE CONFUSED IT WITH 7000 FT ALT. SPECULATION (PLTS): AT THE TIME; WE WERE IN AND OUT OF CLOUDS AND MODERATE TURB. BUT; WE WERE BOTH CONVINCED WE HAD HEARD CORRECTLY. IF OUR ERROR; NEITHER OF US HAD EVER DONE SUCH A THING BEFORE. BTWN US; WE QUESTIONED WHETHER WE HAD ACTUALLY RECEIVED DSCNT TO 7000 FT; THAT IT WAS AN ATC ERROR. RECOMMENDATION: THE ONLY RECOMMENDATION; THOUGH PERHAPS NOT PRACTICAL; IS TO SEGREGATE ALT INSTRUCTIONS FROM ALL OTHER ATC COMS.