AN ACR CAPT REPORTS RUNWAY ASSIGNMENT CONFUSION ON ARRIVAL TO LAX.

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ZLA CLRED US DIRECT GRAMM; CROSS GRAMM AT FL180. PASSING GRAMM; WITHOUT FURTHER CLRNC; WE QUERIED ZLA; WHOSE RESPONSE WAS TO CONTACT SOCAL ON XXX FREQ. WE SWITCHED; WERE CLRED FOR THE CIVET PROFILE; NO RWY SPECIFIED. AS WE ASSUMED WE WOULD GET THE FLT PLAN REQUEST; WE FLEW THE RWY 25L TRANSITION. INSIDE CIVET; NOW IN AN EXPEDITED DSCNT TO MAKE TAROC BELOW 9700 FT DUE TO THE LATE HDOF BY ZLA; WE WERE SWITCHED TO YET ANOTHER SOCAL FREQ; WHO ASKED US WHY WE WERE NOT TRANSITIONING TO RWY 24R. AS PF; I HDG SELECTED TO THE R; AWAY FROM RWY 25L; AND BROUGHT UP THE RWY 24R TRANSITION ON THE FMC. DURING THIS TRANSITION TO RWY 24R; I ALLOWED THE ACFT TO DSND BELOW 9000 FT AT DECOR. UPON RECOGNIZING THE DEV; I DISCONNECTED THE AUTOPLT AND MANUALLY CORRECTED BACK TO 9000 FT. I SUBSEQUENTLY REENGAGED THE AUTOPLT FOR AN UNEVENTFUL APCH AND LNDG. ATC DID NOT COMMENT ON THE ALTDEV; WHICH WAS AROUND 1200 FT. IT IS COMMON TO RECEIVE A VERY LATE CLRNC FOR THE PROFILE FROM ATC; BUT IT SEEMS THERE WAS A MISSED HDOF BY CTR THIS TIME WITH NO OTHER INSTRUCTIONS. AS THE NEXT CTLR CLRED US FOR THE PROFILE (WITHOUT SPECIFYING RWY); AND SUBSEQUENTLY WE WERE SWITCHED TO ANOTHER FREQ; WHO OBVIOUSLY EXPECTED US TO BE ON A DIFFERENT TRACK AND EXPRESSED SO WITH URGENCY; I RUSHED TO COMPLY AND SUBSEQUENTLY MISSED THE SIMILAR ALT OFF THE N SIDE. ABOUT THE ONLY THING I WILL DO NEXT TIME IS TO ASK WHICH TRANSITION TO EXPECT PRIOR TO ARRIVING OVER GRAMM. DON'T FEEL WE RECEIVED MUCH OF ANY SVC FROM EITHER SOCAL OR ZLA; AND FORTUNATELY CONDITIONS WERE VMC.