SMT ON VFR FLT FOLLOWING WITH CENTER IS ADVISED OF ALTIMETER READING DISCREPANCY THAT PLACED ACFT IN PROX OF PCA.

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WHILE ENRTE FROM BULLFROG; UT; TO DENVER (CENTENNIAL); CO; AND AFTER PASSING MONTROSE; CO; I CONTACTED DENVER ARTCC FOR VFR FLT FOLLOWING. THE CTLR; AFTER ACKNOWLEDGING RADAR CONTACT; ASKED THAT I STOP ALT SQUAWK. THE CTLR ASKED ME TO VERIFY 17500 FT AND STATED THAT I SHOWED AN ALT OF 18200. I CONFIRMED MY ALT AND INITIATED A SLIGHT DSCNT. CENTER CONFIRMED THEIR ALTIMETER SETTING OF 30.4? WHICH DIFFERED FROM THE ALTIMETER SETTING I RECEIVED FROM FLT WATCH OVER MONTROSE OF 30.31. THIS ALTIMETER SETTING DISCREPANCY DOES NOT ACCOUNT FOR THE 700 FT DIFFERENCE BTWN BOTH MY ACFT ALTIMETERS AND CENTER'S READOUT OF MY TRANSPONDER ENCODER ALT. THE PITOT STATIC SYS IN THE ACFT WAS BIANNUALED IN FEBRUARY OF 1991. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CALLBACK TO RPTR MADE ACCOUNT SITUATION FAIRLY UNIQUE. RPTR COULD NOT GIVE AN EXPLANATION FOR DIFFERING ALTIMETER READOUTS AND STATED THAT LATER CHKS WITH ATC PROVIDED NO ESSENTIALLY DIFFERENT ALTS. RPTR FELT IT WAS A 1 TIME HAPPENING THAT HE COULD NOT EXPLAIN AWAY. ACFT HAD NOT ENCOUNTERED ANY MOISTURE OR PRECIPITATION OF ANY KIND AND BOTH TRANSPONDERS MODE C WORKED CORRECTLY. NO PROBLEM WITH AGENCY.