APPARENT ALT OVERSHOT ON DESCENT.

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OAK ARTCC HAD GIVEN US DSCNT CLRNC TO FL180 FROM FL310. WE HAD AN SMT AT 17000' OPPOSING US AT 11 O'CLOCK IN SIGHT VISUALLY. WE WERE ON AUTOPLT WITH FL180 ARMED ON THE DFGS. AS WE WERE CAPTURING AND LEVELING AT FL180; OAK ARTCC ASKED IF WE WERE AT FL180. WE CONFIRMED THAT WE WERE. SHORTLY THEREAFTER WE WERE ASKED BY OAK ARTCC TO CONTACT THEM ON THE GND FOR A POSSIBLE ALT DEVIATION. I CALLED OAK ARTCC AFTER LNDG AND TALKED TO THEIR AREA MGR. I WAS TOLD THAT THEIR EQUIP HAD SHOWN US AT 17500' AND CLBING BACK TO FL180. I TOLD HIM THAT WE HAD DSNDED TO FL180; BUT NO LOWER. HE THEN TOLD ME THAT THEIR EQUIP DOES GIVE THEM ERRONEOUS INFO AT TIMES. HE ALSO SAID THAT I COULD CALL THEIR QUALITY ASSURANCE OFFICE ON TUE TO FIND OUT WHAT THEY WILL DO FOR CORRECTIVE ACTION. OUR AIRBORNE EQUIP INDICATED NO ALT DISPARITY AND NO OFF ALT WARNINGS. OUR DFGS FIVES A WARNING AT 250' ABOVE OR BELOW A SELECTED ALT. THERE WAS NO CONFLICT WITH THE SMT AS WE HAD THEM IN SIGHT AND WERE WELL CLR. I FEEL THAT MYSELF AND MY F/O HAVE DONE NOTHING WRONG AND THAT WE WERE AT OUR ASSIGNED ALT OF FL180. OUR ALTIMETERS WERE SET AT 29.92. THE AREA ALTIMETER AT THE TIME WAS 30.02. SUPPLEMENTAL INFO FROM ACN 95122: I HAD SEEN THE CAPT SET AND ARM 18000' ON THE FLT GUIDANCE PANEL. ALL SEEMED UNEVENTFUL UNTIL ATC QUESTIONED OUR ALT AND RPTED A POSSIBLE ALT DEVIATION ON US. OCCASIONALLY THE FLT GUIDANCE SYS WILL NOT CAPTURE EXACTLY THE COMMANDED ALT BUT WILL TRANSITION THROUGH IT TEMPORARILY BEFORE CAPTURING. THIS TRANSITION IS NEVER MORE THAN +/- 100'. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: NO FURTHER CONTACT FROM FAA. DID SAY THE CTLR SAID MIGHT BE AN ALT DISPARITY BUT NO LOSS OF SEP. CERTAIN FLT DID NOT GO BELOW 18000'.