B737 INBOUND TO LAS ON THE TRAGR RNAV 1 ARR MISUNDERSTOOD CLRNC AND MISSED A XING RESTR.

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AFTER FUZZY; CLRED TO DSND VIA TRAGR 1 ARR EXCEPT MAINTAIN 8000 FT. WE STARTED DSCNT AND EVIDENTLY CROSSED POVOC ABOVE 9000 FT. MY UNDERSTANDING WAS THAT WE WERE TO FOLLOW SPD RESTRS BUT VERT NAV WAS AT OUR DISCRETION TO 8000 FT. LESS CONFUSING WORDING WOULD BE 'DSND VIA TRAGR ARR; COMPLY WITH RESTRS; CLRED TO 8000 FT AFTER POVOC.' THE NOTAMS AND CHANGES TO SIDS AND STARS TO LAS VEGAS ARE CONSTANTLY BEING AMENDED AND THE CLRNCS FROM ATC SEEM TO ALWAYS VARY FROM THE STAR/SIDS. I WAS UNDER THE IMPRESSION ONCE THE ALT HAD BEEN AMENDED THE PREVIOUS ALT RESTR WAS REMOVED BUT THIS IS INCORRECT ON MY PART. SUPPLEMENTAL INFO FROM ACN 558698: MY CAPT AND I DISCUSSED WHETHER THE NEXT RESTRS (IPUMY AT 12000 FT MSL AND POVOC AT 9000 FT MSL 190 KTS) WERE STILL REQUIRED. AT POVOC; ATC REQUESTED A CALL AFTER LNDG TO 'GATHER DATA.' MY CAPT CALLED AND THE CTLR SAID WE WERE HIGH AT POVOC. I DO NOT RECALL BEING ABOVE 9000 FT MSL AT POVOC; BUT WE VERY WELL COULD HAVE BEEN AT 10300 FT MSL AT IPUMY. THE CTLR STATED THAT THERE WAS NO VIOLATION; BUT THAT SO MANY FLC'S HAVE BEEN MAKING THE SAME MISTAKE; HE WANTED TO KNOW WHAT COULD BE STATED DIFFERENTLY SO THAT CREWS WOULD FOLLOW THE STEP-DOWNS AND CROSS POVOC AT 8000 FT MSL. AS THE ARR CURRENTLY READS; YOU ARE SUPPOSED TO CROSS POVOC AT 9000 FT AND 210 KTS. THE CLRNC WAS NOT VERY CLEAR.