A320 FLT CREW UNANTICIPATED ZLA CLRNC CHANGE TO SKEBR RNAV STAR CAUSES LATE RECOGNITION TO CORRECT FMS PROGRAMMING TO MEET AMENDED MISEN INTXN XING RESTR.

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WHILE FLYING INBOUND ON THE 'SKEBR RNAV 2 ARR' I WAS HIGH AT THE XING RESTR AT THE MISEN ARR FIX. THE PROB WAS THAT IN THE FMGC ON THE AIRBUS; THE ENTIRE RNAV ARR IS BUILT IN THE BOX AS DEPICTED ON THE PLATE. THAT PLATE ACTUALLY SAYS TO CROSS MISEN AT OR ABOVE FL240. I BELIEVE THAT THE CTLR GAVE US MISEN AT FL240. THE COMPUTER WANTED TO CROSS 'AT OR ABOVE.' BY THE TIME I REALIZED THE ERROR; I HURRIED DOWN TO CROSS MISEN AT FL240. BOTTOM LINE IS THAT I WAS HIGHER THAN FL240 WHEN WE CROSSED. WHILE THIS WOULD BE CONSIDERED PLT ERROR; I BELIEVE THAT THERE ARE SOME PROBS AND/OR CONFUSION OFTEN ASSOCIATED WITH THESE RNAV ARRS. THEY WORK PRETTY WELL WHEN YOU CAN JUST FLY THEM AS THEY ARE DEPICTED (WHICH I BELIEVED WAS THE WHOLE POINT TO THEM). IN REAL LIFE; THEY DON'T USUALLY LEAVE YOU ON THEM. THE CTLRS CHANGE SOME PARTS OF THEM -- USUALLY ALTS -- TO FIT IN SOMEHOW WITH OTHER TFC. CLRING YOU FOR A 'PRE CANNED' ARR THEN LATER CHANGING PARTS OF IT; SEEMS TO ME TO BE SETTING YOU UP TO MISS SOMETHING. I THINK THAT WE SHOULD EITHER FLY THE ARRS AS DEPICTED OR GO BACK TO THE OLD ARRS WITHOUT ALL OF THE PREASSIGNED ALTS.