A DSNDING B737-200 FLC OVERSHOOTS THEIR ASSIGNED ALT OF 11000 FT; GOING TO 10000 FT; 5 NM SW OF MAD; CT.

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DURING DSCNT ON DPK 2 ARR INTO BDL; WHILE DSNDING THROUGH APPROX 10200 FT ON OUR WAY TO 10000 FT; ZBW INQUIRED ABOUT OUR ALT. WE ADVISED WE WERE THROUGH 10200 FT GOING TO 10000 FT. CTR ADVISED ASSIGNED ALT WAS 11000 FT; AND THAT WE SHOULD CLB BACK TO 11000 FT AND SWITCH TO BDL APCH. THERE WAS NO ACFT CONFLICT. THE FO AND I ARE STILL UNSURE HOW THIS HAPPENED. THE ACFT WAS ON AUTOPLT. THE ALT ALERTER WAS SET FOR 10000 FT. WE BOTH BELIEVED WE WERE CLRED TO 10000 FT. ALT AWARENESS PROCS WERE NOT AN ISSUE. TO MY KNOWLEDGE THIS CREW WAS FLAWLESS IN THIS ASPECT DURING THE WHOLE TRIP. MY FO IS AS GOOD AS THEY GET IN OUR COMPANY'S POINT AND VERIFY ALT PROC. POSSIBILITIES: 1) THE CLRNC TO DSND WAS GIVEN IN CONJUNCTION WITH LCL ALTIMETER SETTING OF 30.11. PERHAPS THE CTR CTLR MEANT TO SAY 11000 FT (ONE; ONE THOUSAND) ALTIMETER 30.11 (THREE; ZERO; ONE; ONE) BUT ACTUALLY SAID 10000 FT (ONE; ZERO THOUSAND). WE WILL NEVER KNOW WITHOUT PULLING THE TAPES. 2) THE CLRNC TO DSND WAS GIVEN AS '11000 FT' (ONE; ONE THOUSAND) 'ALTIMETER 30.11' (THREE; ZERO; ONE; ONE) AND BOTH THE FO AND I MISUNDERSTOOD AND READ BACK 10000 FT (ONE; ZERO THOUSAND) WHILE SETTING 10000 FT IN THE ALT ALERTER. ALSO; THE CTLR DID NOT CATCH THE ERROR. 3) THE CLRNC TO DSND WAS GIVEN AS '11000 FT; ALTIMETER 30.11.' IT WAS READ BACK CORRECTLY; BUT SET INCORRECTLY IN THE ALT ALERTER AND BOTH THE FO AND I POINTED TO 10000 FT YET SAID '11000 FT.' WE WILL NEVER (PROBABLY) KNOW WHICH OF THE ABOVE HAPPENED; HOWEVER; A FEW OBSERVATIONS: 1) THE PUBLISHED 'EXPECT' ALT IS 11000 FT. IF THE ASSIGNED ALT WAS NOT 11000 FT WE COULD HAVE QUESTIONED THIS. 2) EXTRA CARE SHOULD BE TAKEN WITH 10000 FT (ONE; ZERO) AND 11000 FT (ONE; ONE) CLRNCS AS THEY HAVE A HIGHER TENDENCY TO BE ISSUED; HEARD; READ BACK AND SET INCORRECTLY.