B737 FLC DSNDS BELOW ASSIGNED ALT; EVEN THOUGH BOTH PLTS BELIEVE OTHERWISE. FLC BELIEVES COCKPIT PROCS WERE CORRECT. NO ATC CORRECTIVE ACTION PERCEIVED.

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WE WERE AT CRUISE AT FL330. I WAS FLYING AS FO AT CTLS; FMS NAVING; AUTOPLT WAS ENGAGED. ATC GAVE US A CLRNC TO DSND TO FL280. THE CAPT READ BACK THE DSCNT AND POINTED TO THE ALT ALERTER THAT I HAD DIALED IN TO CORRESPOND TO 28000 FT. I SELECTED FL280 IN THE CRUISE PAGE OF THE FMC; EXECUTED THE COMMAND AND THE AUTOPLT STARTED OUR 1000 FPM RATE OF DSCNT. AT THIS PHASE OF THE FLT; NEITHER THE CAPT NOR MYSELF HAD ANY COGNITIVE REASON TO EXPECT ANY SPECIFIC ALT TO DSND TO. AS WE DSNDED THROUGH 28500 FT; THE CTLR STATED 'ACR X; VERIFY ASSIGNED ALT FL290.' THE CAPT STATED 'NEGATIVE;' ASSIGNED FL280. CTLR STATED TO MAINTAIN FL290. I IMMEDIATELY SET UP A 2000 FPM CLB IN VERT SPD MODE OF AUTOPLT BACK TO FL290. THERE WAS NO CONFLICT NOTICED BY EITHER THE CAPT OR MYSELF ON TCASII; NOR DID THE CTLR EVER STATE A CONFLICT HAD OCCURRED. WE THEN QUESTIONED THE CTLR AS TO WHAT WAS OUR ORIGINAL ASSIGNED ALT; AND HE STATED THAT HE HAD FL290 IN 'HIS BOX' AND THAT IS WHAT HE WOULD HAVE TO SAY WAS CORRECT. BOTH THE CAPT AND I BELIEVE WE WERE ASSIGNED FL280. SUPPLEMENTAL INFO FROM ACN 397500: AS THE CREW; WE DISCUSSED THE ISSUE. WE HAVE NO REASON TO BELIEVE THERE WAS A PROB WITH OUR PROCS OR UNDERSTANDING OF THE CLRNC.