B737 CREW HAD RWY INCURSION ON TAXI OUT.

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ON TAXI OUT TO RWY 15; GND CTLR ASKED WHY WE TAXIED PAST RWY 21. WE THOUGHT WE WERE CLRED TO RWY 15 FOR TKOF. APPARENTLY GND HAD CLRED US TO RWY 21 AND WE 'THOUGHT' IT WAS TO RWY 15 BECAUSE: WE LANDED ON RWY 15. RWY 15 WAS IN CONTINUAL USE DURING OUR TURN AROUND AT THE GATE. OUR WT AND BAL FIGURES WERE FOR RWY 15. WAS CLRED TO TAXI TO RWY 15 DURING OUR GND OP. GND CTL'S ORIGINAL INSTRUCTION TO US FADED OUT AND THE PART WE MISSED (WHEN IT WAS READ BACK) TURNED OUT TO BE 'HOLD SHORT OF RWY 15;' SO WE WERE FOCUSED ON THAT. RWY 15 OR RWY 33 WAS THE EXPECTED DEP RWY SINCE IT IS 9000 FT LONG VICE 7000 FT LONG FOR RWY 21 (WHICH IS NORMALLY NOT USED) AND THE WIND WAS VARIABLE AT 4 KTS. WE JUST DIDN'T HEAR '21.' NO CONFLICT RESULTED; NO EVASIVE ACTION WAS REQUIRED. SUPPLEMENTAL INFO FROM ACN 419600: GND CTLR ISSUED TAXI INSTRUCTIONS TO US TWICE BECAUSE BOTH MYSELF AND FO DID NOT RECEIVE CLRLY. SOMETHING WAS LOST ON EACH OF THE XMISSIONS. THE CTLR SAID WE WERE CLRED TO RWY 21; BUT WE DID NOT HEAR AT ALL OF CLRLY ON READBACK.