ALT DEVIATION; WRONG ALTIMETER SETTING.

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ENRTE CMH-DTW; FAIRLY SHORT LEG. F/O FLYING. I DID IN RANGE CHKLIST WHICH CALLS FOR PLTS TO VERBALLY STATE SETTING AND XCHK EACH OTHER. I STATED 29.85 AND XCHKED HIS. HE DID THE SAME. I WAS CALLING IN RANGE TO COMPANY IN DTW WHEN ATC ASKED OUR ALT. HE ANSWERED THEM; '10000';' ASSIGNED ALT. THEY SAID; 'CHK YOUR ALTIMETERS.' I THEN LOOKED AT THE SHEET WHERE I WROTE THE ATIS INFO DOWN. ALTIMETER SETTING THAT I WROTE DOWN WAS 28.85; BUT WE BOTH HAD SET 29.85. CONSEQUENTLY WE WERE CRUISING AT 9000' BUT INDICATING 10000'. WE IMMEDIATELY CLBED UP. ATC SAID NOTHING ELSE. NEITHER DID WE. IT IS A FAIRLY RUSHED LEG. I THINK WE WERE KEEPING OUR SPD UP FOR ATC. I DID THE INRANGE CHK AS SOON AS WE BEGAN OUR DSCNT FROM 22000'. I MUST HAVE SIMPLY MISSTATED THE UNUSUALLY LOW ALTIMETER SETTING TO ONE MORE NORMAL. IN CMH THE ALTIMETER SETTING WAS 29.02 OR SOMETHING LIKE THAT. SO THE CHANGE WAS MARKED FROM 29.92; BUT NOT SO MUCH FROM CMH. NEITHER THE F/O NOR MYSELF BELIEVE IT HAPPENED BECAUSE I RUSHED THE CHKLIST. I SAID LOUDLY AND POINTEDLY WHAT I THOUGHT IT WAS--AND CHKED TO SEE WE SET IT. I THINK IT HAPPENED BECAUSE I DID NOT RECOGNIZE THE UNUSUAL NATURE OF THE SETTING AND REVERTED TO MORE FAMILIAR SETTINGS DURING THE CHKLIST. THIS FAILURE TO RECOGNIZE MAY HAVE BEEN CAUSED BY HIGH WORKLOAD AT TIME AND ALSO AN UNDERDEVELOPED APPRECIATION FOR ALTIMETER SETTINGS IMPORTANCE DUE TO RARITY OF SUCH VAST DIFFERENCES. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR STATES BOTH HE AND F/O SHOCKED WHEN REALIZED MISTAKE. HAD NOT RECOGNIZED EXTREME LOW PRESSURE WHEN HEARD ATIS. IN BRIEFING LEARNED THAT DTW WX WAS OK; JUST HIGH OVCST. DID NOT PAY MUCH ATTN TO DTW; PUT FOCUS ON STL WHERE KNEW WOULD HAVE TO MAKE INSTRUMENT APCH. HEARD SETTING CORRECTLY; WROTE IT DOWN CORRECTLY AND SIMPLY SET ALTIMETER WITH MORE NORMAL NUMBERS.