A CLBING B727-200 FREIGHTER OVERSHOOTS ITS ASSIGNED ALT WHEN THE ALT CAPTURE DOES NOT PERFORM AS EXPECTED NEAR ICT; KS.

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DURING CLBOUT ON A FLT FROM ICT-MEM WE WERE TOLD TO LEVEL OFF AT FL270 FOR XING TFC AT FL280. AFTER LEVELOFF; WE NOTICED A 200 FT DIFFERENCE BTWN THE CAPT'S AND FO'S ALTIMETERS WHICH PUT US ABOUT 200 FT HIGHER THAN ASSIGNED. WE REALIZED THE ERROR AND MADE AN IMMEDIATE CORRECTION BACK TO FL270. SHORTLY THEREAFTER; CTR ASKED US TO CONFIRM OUR ALT; BY WHICH TIME WE HAD ALREADY RETURNED TO FL270. HE SAID THAT HE INDICATED A TEMPORARY 200 FT DEV. WE LATER DETERMINED THAT THE ALTIMETERS WERE WITHIN TOLERANCE. SUPPLEMENTAL INFO FROM ACN 449175: AS THE ACFT APCHED FL270; AN ALT SPLIT HAD OCCURRED AND GONE UNNOTICED; WITH THE CAPT'S ENCODING ALTIMETER READING APPROX 200 FT HIGHER THAN MINE (PF). INITIAL AUTOPLT FUNCTIONS/INDICATIONS WERE NORMAL AS WE APCHED FL270. THE MACH HOLD SWITCH WENT TO 'VERT SPD' AND THE A 'ALT SELECT' LIGHT WENT FROM AMBER TO GREEN. NORMALLY; RATE OF CLB BEGAN TO DECREASE FROM APPROX 1500 FPM. THE AUTOPLT DID NOT GO TO 'ALT HOLD' (GREEN LIGHT); AND I LOOKED OVER TO THE CAPT'S ENCODING ALTIMETER AS MY ALTIMETER INDICATED FL270.5. WHEN I SAW THE CAPT'S ALTIMETER AT FL272.5; I PUT CORRECTIVE 'NOSE DOWN' INPUT ON THE AUTOPLT 'VERT SPD WHEEL' GETTING BACK TO FL270 ON THE CAPT'S ENCODING ALTIMETER WITHIN APPROX 5 SECONDS. I INADVERTENTLY MISSED A SIGNIFICANT ALTIMETER SPLIT. THE SIT REQUIRED IMMEDIATE ACTION DUE TO XING TFC. THIS WAS MY FIRST FLT (ACTUAL) AFTER HAVING BEEN OFF FOR OVER 1 MONTH. I HAD JUST COMPLETED ANNUAL RECURRENT TRAINING THE WK BEFORE.