ALTIMETER READOUT DISAGREES BTWN ACFT AND ARTCC CTLR.

Narrative

ACFT ENRTE FROM PHX TO TYS CLBING TO FL390. ZAB GAVE (3) 10 DEG TURNS (TOTAL OF 30 DEGS) TO THE L OF COURSE DUE TO TFC OVERTAKING US. ZAB THEN GAVE CLRNC TO PROCEED DIRECT TO IFI VOR AFTER REACHING FL390. WHEN AUTOPLT LEVELED AT FL390; OUR COURSE WAS CHANGED TO DIRECT IFI. AFTER SEVERAL ATC FREQ CHANGES; AND STILL WITH ZAB; WE WERE GIVEN A TELEPHONE NUMBER TO CALL AT OUR CONVENIENCE FOR ZAB. AT THAT TIME WE DID NOT KNOW THE REASON FOR THE CALL. UNABLE TO REACH OUR PARTY WHILE ENRTE; ANOTHER CALL WAS MADE AFTER REACHING TYS. WHILE SPEAKING TO MR X OF ZAB WE DISCOVERED THAT ZAB RADAR WAS SHOWING OUR ACFT AT FL384 WHEN WE RPTED LEVEL AT FL390 AND CHANGED COURSE FOR IFI. AT THAT POINT HE SAID THEY LOST PROPER SEPARATION WITH OTHER TFC. OUR EQUIP SHOWED US TO BE AT FL390 AND WE HAD NO PREVIOUS COMPLAINTS PRIOR TO THIS LEG; NOR HAVE WE HAD ANY SINCE. IT HAS ALWAYS BEEN THE PROC BY CTR IF AN ALT IS NOT AS IT SHOULD BE; THEY QUERY THE ACFT. IN OUR SIT; NONE WAS MADE AT ANY TIME; THAT I CAN RECALL.