NMAC DURING A CIRCLING APCH.

Narrative

LNDG AT MDW DURING GOOD VMC WX. AS USUAL RUNNING PARALLEL OPS. I THINK THEY WERE DOING THE ILS RWY 31C CIRCLE RWY 22L. I WAS ON A STRAIGHT-IN FINAL FOR RWY 22R. THE B-757 WAS ON A L DOWNWIND FOR RWY 22L. WE WERE EACH GIVEN AS TFC. I RPTED THE 757 IN SIGHT -- HE DID NOT SEE US. THERE WAS A STRONG WIND FROM THE S AND THE 757 APPEARED LIKE HE WAS GOING TO OVERSHOOT THE FINAL. I STARTED WIDENING OUT TO THE R AND BY THIS TIME WAS WELL N OF THE RWY 22R CTRLINE. THE 757 ROLLED OUT OF HIS TURN AND WAS NOW COMING STRAIGHT AT ME (HE WAS APPROX PERPENDICULAR TO THE RWY CTRLINE). HE THEN ADVISED HE WAS DEVIATING AND CLBING PER TCASII ALERT. HE CONTINUED CLBING AND WENT OVER THE TOP OF MY ACFT. BOTH THE CTLR AND I WERE PUZZLED AS TO WHAT THE HECK HE WAS DOING AS THERE WAS NO CONFLICT UNTIL HE CREATED ONE. IF HE HAD ROLLED OUT ON FINAL AS PER HIS CLRNC IT WOULD HAVE BEEN ROUTINE TFC. MY 'GUESS' IS THAT HE REALIZED HE WAS GOING TO OVERSHOOT AND CLBED TO AVOID ME. REGARDLESS; I BELIEVE THAT TCASII INFO SHOULD BE TAKEN WITH A GRAIN OF SALT WHEN IN THE TFC AREA. IT IS DANGEROUS TO INITIATE AVOIDANCE WHEN IT ISN'T WARRANTED. ESPECIALLY WHEN MULTIPLE RWYS ARE IN USE; WHICH IS THE NORM AT MOST ARPTS THAT TCASII EQUIPPED ACFT OPERATE OUT OF. HE KNEW THERE WAS TFC FOR THE PARALLEL AND THAT I HAD VISUAL CONTACT WITH HIM. I JUST HOPE THAT AS HE RE- ENTERED THE TFC PATTERN HE DIDN'T TELL THE PAX THAT 'ONE OF THEM LITTLE PLANES GOT IN OUR WAY.' LET'S PAY ATTN OUT THERE!