ALTIMETER MALFUNCTION RESULTS IN FLT CREW LEVELING ACFT 300' LOW.

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VECTORS FOR APCH AT ORD. WE WERE TOLD TO DSND TO 7000' MSL. 7000' WAS SET IN THE ALT SET WINDOW AND FLT LEVEL CHANGE WAS ACTIVATED ON THE AFDS PANEL. AFTER LEVELING AT 7000' WE NOTICED THE CENTER ALTIMETER READ ABOUT 7300'. WE RECHKED THE ALTIMETER SETTINGS; VERIFIED THEY WERE CORRECT AND DECIDED THAT FOR SOME REASON THE COMPUTER HAD LEVELED US OFF 300' HIGH. WE THEN DSNDED TO 7000' MSL ON THE CENTER ALTIMETER. THE ALTIMETERS IN FRONT OF EACH PLT HAD PREVIOUSLY BEEN SET TO QFE AS PER SOP. SHORTLY THEREAFTER; APCH CTL ADVISED US THAT WE WERE AT 6800' MSL AND TO CHK OUR ALT. I REPLIED TO THEM THAT OUR ALTIMETER SAID ABOUT 6950'. WE THEN DECIDED THE CENTER ALTIMETER WAS IN ERROR AND THAT THE COMPUTER HAD ACTUALLY LEVELED US OFF AT THE CORRECT ALT THE FIRST TIME. WE THEN CLBED BACK UP TO 7300' MSL INDICATED ON THE CENTER ALTIMETER. ON THE GND ALL 3 ALTIMETERS CHKED OUT WELL WITHIN TOLERANCES; BUT A MAINT DISCREPANCY WAS ENTERED IN THE ACFT LOG BOOK. WE SHOULD HAVE ASKED APCH CTL WHAT THEY SHOWED BEFORE WE DSNDED; BUT IT'S PRETTY HARD TO SQUEEZE A WORD IN WHEN WORKING ORD APCH.