BAE146 IS QUESTIONED BY ATC REF ALT DURING PARALLEL APCHS INTO ORD.

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WHILE BEING VECTORED FOR THE ILS RWY 9R APCH TO ORD; AT 7000 FT MSL; CHICAGO APCH CTL CLRED US FOR THE APCH 'TO CROSS PRATT AT 5000 FT.' AS PF; I COMMENCED THE DSCNT IMMEDIATELY DUE TO TURB AT THAT ALT AND EXPEDITED OUR DSCNT WITH A HIGHER THAN NORMAL DSCNT RATE. WE WERE OUTSIDE OF THE PRATT FIX AND THE GS HAD NOT STARTED TO COME 'ALIVE' WHEN INTERCEPTING THE LOC. ORD WAS ACCEPTING PARALLEL APCHS AT THIS TIME TO BOTH RWYS 9L AND 9R. THE INITIAL INTERCEPT ALT FOR WY 9L; IS AT 4000 FT MSL; WHILE RWY 9R (OUR RWY) 5000 FT MSL. AS WE INTERCEPTED THE LOC; WE OBSERVED THE WINDS ALOFT ON OUR FMS INDICATED 180 DEGS AT OVER 50 KTS. OUR ASSIGNED HEADING WAS 180 DEGS. APPROX 2 MI OUTSIDE OF PRATT; ATC TOLD US TO 'TURN TO 120 DEGS AND INTERCEPT;' WHICH WE DID. WHILE IN MY TURN TO THE ASSIGNED HEADING OF 120 DEGS; WE OBSERVED THAT WE RAPIDLY PASSED THROUGH THE LOC. I IMMEDIATELY ROLLED WINGS LEVEL AND CORRECTED BY TURNING BACK TO A MORE SOUTHERLY HEADING. MY CONCERN; OF COURSE; WAS THE PARALLEL APCHS. IN MY OPINION; THE ATC ASSIGNED HEADING AND OUR PROX TO THE LOC WAS INSUFFICIENT TO ALLOW US TO INTERCEPT WITHOUT GOING THROUGH THE LOC. REGARDLESS; THE ACFT WAS QUICKLY CORRECTED AND STABILIZED ON THE LOC COURSE. WE WERE STILL DSNDING RAPIDLY AND LEVELED AT 5000 FT MSL; AS ASSIGNED; WHEN ATC ASKED OUR ALT. THE CAPT (PNF) RESPONDED '5000 FT AS ASSIGNED.' THE APCH CTLR ADMONISHED US BY SAYING 'YOU'RE AT 4500 FT...YOU WERE ASSIGNED 5000 FT.' 'HUH; WHAT'S HE TALKING ABOUT?' WE HAD LEVELED AT 5000 FT MSL. AS WE ARRIVED OVER PRATT AT 5000 FT; THE GS INTERSECTED OUR ALT AND WE FOLLOWED IT DOWN TO A NORMAL LNDG. LATER THAT EVENING; I TELEPHONED THE TRACON AND SPOKE WITH THE WATCH SUPVR WHO ASSURED ME THERE HAD BEEN 'NO PROB.' I DON'T KNOW WHAT HAPPENED HERE. PERHAPS A COMBINATION OF A COUPLE THINGS MIGHT HAVE BEEN IN PLAY HERE. THERE IS A PHENOMENON I HAVE EXPERIENCED IN THE PAST EXPLAINED TO ME AS 'PROFILE SHADOW' WHEREBY AN ACFT IN A HIGH RATE OF DSCNT LEVELS OUT AT THE ASSIGNED ALT; HOWEVER; THE ATC COMPUTERS' DETECTION RATE CANNOT UPDATE QUICKLY ENOUGH TO REFLECT AND SYNCHRONIZE THE ALT WITH THAT SUPERIMPOSED ON THE MODE C TRANSPONDER SIGNAL. THE ATC COMPUTERS CONTINUE TO 'THINK' THE ACFT IS DSNDING AT THE HIGH RATE; AND THEY SHOW AN ERRONEOUS ALT ON THE FIRST COUPLE SWEEPS; UNTIL ALL COMPONENTS GET SYNCHRONIZED WITH THE ACFT. IT IS MY UNDERSTANDING THAT THIS PHENOMENON CAN EVEN CAUSE AN 'ALT ALERT' ON THE ATC SCREEN. ANOTHER POSSIBILITY MIGHT BE THE FACT THAT THE CTLR GAVE US A HEADING THAT SHOT US THROUGH THE LOC EARLY IN OUR TURN. THERE MAY HAVE BEEN SOME CONFUSION WITH US AND THE ALT OF OTHER ACFT FLYING THE APCH TO RWY 9 AS HE; TOO; DSNDED ON HIS GS. REGARDLESS; WE AS A CREW ARE GRATEFUL THAT ATC CALLED THIS OBSERVED (ALTHOUGH INCORRECT) EXCURSION TO OUR ATTN. OBVIOUSLY A FALSE ALT WARNING SUCH AS THIS WHEN ALL IS WELL IS BETTER THAN THE CONSEQUENCES FROM NO WARNING AT ALL IN A REAL EXCURSION SIT.