ACCEPTING ANOTHER'S CLRNC LEADS TO AN ALT OVERSHOOT ON ARR TO LAS; NV.

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UPON DSCNT INTO LAS; ZLA HAD CLRED US TO CROSS CLARR AT 13000 FT AND 250 KTS. WHILE PASSING OVER SKEBR; CTR SWITCHED US TO APCH CTL. WHILE CHKING IN; I THOUGHT I UNDERSTOOD THE APCH CTLR DIRECT US TO DSND TO 8000 FT. I DIALED IN THE ALT AND THE FO CONFIRMED IT AND BEGAN TO LEAVE 13000 FT. WE WERE SWITCHED TO ANOTHER FREQ BEFORE I WENT OFF TO GET OUR GATE ASSIGNMENT. THE FO RPTED 'NO CHANGES' WHEN I CAME BACK ON THE FREQ. IT WAS ABOUT THAT TIME THAT APCH ASKED US WHAT ALT WE WERE DSNDING TO -- WE WERE PASSING 11000 FT. I SAID THAT OUR CLRNC WAS TO 8000 FT. HE THEN INFORMED US THAT WE HAD TAKEN SOMEONE ELSE'S CLRNC AND DIRECTED US TO STOP THE DSCNT AT 10000 FT. THE REST OF THE FLT WAS UNEVENTFUL. I MUST ADMIT THAT I'M AT A LOSS TO EXPLAIN EXACTLY WHY THIS HAPPENED SINCE WE BOTH UNDERSTOOD TO HAVE HEARD THE SAME THING. THIS EVENT STRESSED TO ME THAT IT IS ESPECIALLY IMPORTANT TO REMAIN EXTRA VIGILANT DURING THE APCH PHASE OF FLT. WE MIGHT HAVE QUESTIONED THE CTLR UPON SETTING 8000 FT IN THE ALT WINDOW SINCE THERE IS A HIGHER RESTR (12000 FT) FURTHER ALONG THE STAR AT IPUMY. HAD WE DONE THIS; OUR MISUNDERSTANDING WOULD HAVE BEEN RESOLVED. SUPPLEMENTAL INFO FROM ACN 561483: I WAS PF ON THE SKEBR RNAV ARR AT 250 KTS; 13000 FT INTO LAS PASSING CLARR AT 250 DEGS; 13000 FT. OVER SKEBR TURNING; ATC ISSUED AN 8000 FT ALT. WE WERE NOT TOLD TO DSND VIA THE STAR; WE WERE ASSIGNED ALTS. CAPT DIALED IN 8000 FT AND I VERIFIED AND BEGAN DSCNT TO 8000 FT. ATC GAVE US A FREQ CHANGE. CAPT FIRST CHANGED THE FREQ AND CHKED IN WITH ATC OF OUR ALT ASSIGNMENT AND OUR PHASE WITHIN OUR DSCNT. ATC'S RESPONSE WAS THAT OF A NORMAL CHK-IN BY AN ACKNOWLEDGE.