SMA PLT USES ALTIMETER SETTING GIVEN BUT IS BELOW ASSIGNED ALT AND RECEIVES CHANGE OF ALTIMETER SETTING WITHIN 25 MI. HAS NMAC.

Narrative

I ENTERED THE PHX TCA AT CASA GRANDE ARPT APPROX 30 MI S OF PHX AT 4500 FT. I WAS CLRED INTO THE TCA AND TOLD TO MAINTAIN 3500 FT. I ASKED FOR AN ALTIMETER SETTING AND RECEIVED 29.92. WHEN I WAS APCHING MEMORIAL ARPT; APCH ADVISED OF TFC FLYING W TO E AT 3100 FT (TFC WAS NOT IN CONTACT WITH APCH CTL). AS THE TFC APCHED I WAS ADVISED BY APCH CTL OF A TA. BY THIS TIME TFC WAS IN SIGHT AND FLEW UNDER ME. AS I GOT CLOSER TO AHWATUKEE (TOWN) I WAS AGAIN ADVISED OF AN SMT XING MY FLT PATH AT 3100 FT. THE SMT WAS ADVISED THAT I WAS AT 3500 FT. I SAW THE SMT AND NO CONFLICT OCCURRED. I WAS THEN HANDED OFF TO APCH ON 120.7 AND GIVEN A NEW ALTIMETER SETTING OF 29.84. AS I WAS PUTTING IN THE NEW SETTING I WAS ADVISED OF AN SMA AT 3100 FT OPPOSITE DIRECTION. THE CTLR THEN TOLD ME I WAS SHOWING 3400 FT ON HIS SCOPE. I ASSUME THAT WAS CAUSED BY ME CHANGING AT THAT TIME TO 29.84 FROM 29.92. I THEN CALLED HIM BACK AND HE SAID I WAS NOW INDICATING 3500 FT. HE ALSO SAID THAT AT ONE TIME THE READING WAS AS LOW AS 3300 FT; AND REMARKED I WAS WITHIN TOLERANCE; AND SAID NO MORE UNTIL HE TERMINATED CTL. I AM CONCERNED WITH WHY 2 DIFFERENT ALTIMETER SETTINGS IN LESS THAN 25 NM; AND WHY THEY HAD ME AT 3500 FT; 30 NM OUT WHEN THEY NORMALLY HAVE YOU AT 4500-6500 FT. I SHOULD HAVE QUESTIONED THE ALT; SINCE I HAVE HAD 2 NEAR MISSES AT THE SAME LOCATIONS BEFORE; WHEN UNDER THEIR CTL; REQUIRING EXTREME EVASIVE MANEUVERS. THE TFC BOTH TIMES WERE NOT TALKING TO PHX APCH AND APCH DID NOT ADVISE ME OF THEIR PRESENCE.