A320 FLC EXCEEDS AIRWAYS BOUNDARIES DURING STAR INTO DTW.

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WE WERE A FLT FROM PHX TO DTW. WE WERE GIVEN THE MIZAR 3 ARR OVER LFD. THE FLT WAS APCHING LFD IN A DSCNT TO CROSS MIZAR AT 12000 FT. APPROX FLT ALT WAS IN THE MID 20'S. THE ACFT SPD WAS ABOUT 315 KIAS WITH A LARGE TAILWIND. THE A320 COMPUTED A TURN RADIUS THAT EVENTUALLY TURNED THE ACFT TO MISS LFD BY ABOUT 5 MI. OUTBOUND ON THE ARR; THE CTLR (IN A RATHER DRY MANNER) ASKED US WHAT OUR ROUTING WAS. WE REPLIED THAT IT WAS THE MIZAR 3 OVER LFD. THE CTLR THEN INFORMED US THAT WE HAD MISSED LFD BY 5 MI. WITH LITTLE HESITATION; THE FO RESPONDED THAT WE HAD OVERFLOWN LFD. AFTER SOME DISCUSSION BTWN THE FO AND MYSELF; WE DECIDED NOT TO DISCUSS WHO WAS RIGHT OR WRONG IN THE MATTER OVER THE RADIO. SHORTLY AFTERWARDS; THE CTLR VECTORED US OFF THE ARR FOR A FEW MI AND THEN HAD US FLY DIRECT TO MIZAR. AS FAR AS I KNOW; THERE WERE NO TFC CONFLICTS. THE CTLR SEEMED PERTURBED; BUT NEVER BROUGHT UP THE SUBJECT AGAIN. AFTER THE TRIP; I BROUGHT UP THE ISSUE WITH ONE OF OUR A320 CHIEF PLTS. HE INFORMED ME THAT THE ISSUE IS VAGUE. I THEN LOOKED UP THE SUBJECT IN THE ONLINE AIM. HERE; TOO; THE ISSUE IS CONVOLUTED AS LEADING TURNS ARE EXPECTED TO HELP KEEP THE ACFT WITHIN THE 4 NM LIMIT OF THE AIRWAY. BUT THIS ONLY APPLIES BELOW FL180. ABOVE FL180 THERE APPEARS TO BE NO SUCH LIMIT. ADDITIONALLY; WE WERE NOT ON AN AIRWAY AS WE WERE CLRED DIRECT TO LFD TO BEGIN THE ARR. MANY OTHER A320 CREW MEMBERS HAVE HAD SIMILAR SITS. THE AIRBUS FMGS SOFTWARE PLANS FOR A 25 DEG BANK TURN AT 1G AND CAN AND DOES MAKE LARGE LEADING TURNS -- ESPECIALLY AT HIGH ALT WITH A TAILWIND. BECAUSE OF THIS; THE FOM STATES THAT CREWS MAY HAVE TO MAKE ADDITIONAL FLT PLAN INPUTS SO AS TO HELP THE ACFT REMAIN WITHIN THE AIRWAY LIMITS IF THOSE LIMITS APPLY. I FIND THIS ODD; AS THE A320 ACFT AND ITS SOFTWARE ARE CERTIFIED TO OPERATE IN UNITED STATES AIRSPACE. NEVERTHELESS; IN THE FUTURE; I WILL TRY TO ASSIST THE ACFT IN ITS FLT PATH ANY TIME SUCH ACFT DERIVED FLT PATH MAY BE QUESTIONED.