A B737-300 CREW; DSNDING INTO LAS; UNDERSHOT PUBLISHED STAR ATC ASSIGNED ALT.

Narrative

I WAS PLTING THE ACFT ON A FLT FROM LAX TO LAS. THE RTE TOOK US FROM DAGGETT ON THE SKEBR 1 ARR. CTR CLRED US TO CROSS MISEN AT FL240. WE WERE AT FL290 AT THE TIME. I HAD PROGRAMMED THE 'BOX' FOR THE ARR; BUT MISSED THE FACT THAT THERE WAS AN 'A' PREPROGRAMMED INTO THE RTE AT MISEN; IE; MISEN 240A. SO; ABOUT 6 MI FROM MISEN; WE GOT ASKED TO START DOWN; SO WE DID. WE CROSSED MISEN AT ABOUT FL260. NEEDLESS TO SAY; A CLOSER EXAMINATION/PREFLT OF THE ARR WOULD HAVE SOLVED THE MISSED ALT RESTR REQUEST BY ZLA. SUPPLEMENTAL INFO FROM ACN 541161: I HAD A FEELING THAT SOMETHING WAS NOT RIGHT WITH THE FMC SETUP; BUT WAS UNABLE TO FIGURE OUT WHAT IT WAS. IT SEEMS TO ME THAT I HAVE READ ABOUT THIS PROB IN THE PAST; BUT COULD NOT REMEMBER THE SPECIFICS. I TRIED TO LOOK AT THE LEGS PAGE TO SEE THE ALT THAT THE BOX WAS SHOWING AT MISEN. I SAW '240A' -- UNFORTUNATELY; I DIDN'T REALLY UNDERSTAND THE SIGNIFICANCE OF THE 'A.' I WENT OFF THE ATIS AND UPON RETURNING; ZLA ASKED IF WE HAD STARTED DOWN FOR MISEN YET. I SAID NO AND THEN NOTICED WE WERE ONLY 1.5 MI FROM MISEN. WE STARTED DOWN AND WERE HANDED OFF TO THE NEXT SECTOR. A MISUNDERSTANDING OF THE PREPROGRAMMED ALT AND THE EFFECT OF THE 'A' NEXT TO IT LED US TO BELIEVE THE DSCNT PAGE WOULD KEEP US WHERE WE NEEDED TO BE. A BETTER UNDERSTANDING OF THE SYS FROM THIS EXPERIENCE SHOULD PRECLUDE IT FROM HAPPENING AGAIN AT LEAST TO THIS CREW.