AN MD80 FLC GETS CONFUSED OVER THE PHRASEOLOGY AS USED BY THE ZMA CTLR AND FLIES DIRECT TOWARDS LA BELLE WHEN THEY HAD ACCEPTED A HDG OF 325 DEGS OVER THNDR INTXN; FL.

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CLRNC RECEIVED FROM FLL CLRNC DELIVERY WAS 'CLRED AS FILED; EXPECT FLL 8 DEP; THNDR TRANSITION; DIRECT LA BELLE; SRQ; FLT PLAN RTE.' AFTER TKOF ON RWY 9L; WE WERE GIVEN SEVERAL HDG CHANGES AND ALT CHANGES. WE WERE THEN INSTRUCTED TO JOIN THE DHP 335 DEG RADIAL TO THNDR. WE WERE ADVISED OF TSTMS IN THE AREA. WE ASKED TO FLY SLIGHTLY L AND WERE APPROVED TO DEVIATE L OF COURSE; 'NO MORE L THAN A HDG OF 310 DEGS. ADVISE WHEN ABLE TO FLY A 325 DEG HDG.' WE DEVIATED AS NECESSARY AND WHEN WE WERE CLR OF THE TSTMS; I INSTRUCTED THE FO TO ADVISE ZMA THAT WE COULD ACCEPT A 325 DEG HDG. HIS RESPONSE WAS 'ROGER.' AT THAT POINT WE WERE APCHING THNDR (DHP 335 DEGS AT 57 DME). I ASKED THE FO TO CONFIRM THAT OUR CLRNC WAS 'THNDR; DIRECT LA BELLE; SRQ; FLT PLAN RTE.' MIA RESPONDED BY CONFIRMING OUR RTE OF FLT AND SAID; 'EXPECT DIRECT SRQ IN ABOUT 15 MI.' WHEN WE REACHED THNDR; WE TURNED L DIRECT LA BELLE. SOON AFTER; MIA ASKED US WHAT OUR HDG WAS. WE RESPONDED THAT IT WAS ABOUT 300 DEGS; AFTER WHICH THE ZMA CTLR SAID OUR ASSIGNED HDG WAS 325 DEGS AND 'TURN R TO 340 DEGS IMMEDIATELY; YOU HAVE COMPANY ACFT 8 MI OFF YOUR L.' FIRST; OUR LAST CLRNC WAS TO JOIN THE THNDR TRANSITION. SECOND; WE ADVISED CTR (AS HE REQUESTED) WHEN WE COULD FLY A 325 DEG HDG. WE WERE NEVER INSTRUCTED TO MAINTAIN A 325 DEG HDG. WITH RECENT FAA INTERPS OF READBACK ERRORS BEING PLACED ENTIRELY ON THE PLT; IT SEEMS THAT THE FAA CTLRS ARE BECOMING MORE AND MORE LAX ON USING PROPER PHRASEOLOGY. THEY SEEM TO REALIZE THAT THEY WILL BE EXONERATED FROM ANY ERRORS THEY MAY COMMIT BECAUSE THE PRESUMPTION IS THAT THE PLT WAS IN ERROR. IN THIS CASE; ZMA WAS CLRLY IN ERROR. SUPPLEMENTAL INFO FROM ACN 472137: HIS POOR TECHNIQUE LED TO CREW CONFUSION ABOUT THE CLRNC. THE LAST CLRNC WE RECEIVED WAS 'INTERCEPT THNDR TRANSITION; FLT PLAN RTE' (PRIOR TO VECTORS FOR WX).