PLT PROGRAMS CORRECT ALT INFLT MGMNT COMPUTER AND WRONG ALT IN MODE CTL PANEL AND DEFAULTS TO WRONG ALT.

Narrative

IN CRUISE AT FL350; ATC CLRED US TO CROSS JAXSN AT FL330. A FEW MINS LATER ATC RECLRED US TO CROSS 15 NM S OF JAXSN AT FL330. I WROTE DOWN THE CLRNC ALT AND DISTANCE. THEN SET 31000 FT IN THE MODE CTL PANEL (B757); BUILT THE FIX AND ENTERED FL330 IN THE FMS. AS WE APCHED THE ASSIGNED XING FIX I GLANCED AT THE 31000 FT I HAD ENTERED IN THE MODE CTL PANEL AND DETERMINED WE WOULD HAVE TO INCREASE OUR RATE OF DSCNT TO COMFORTABLY MAKE FL310. THE FMS SHOWED US LOW ON THE DSCNT PROFILE BUT I IGNORED IT AS THEY CAN OCCASIONALLY BE OFF. THE ACFT TRIED TO LEVEL AT FL330 IN VNAV BUT I WAS CONVINCED OUR ASSIGNED ALT WAS FL310. I SELECTED FLT LEVEL CHANGE ON THE MODE CTL PANEL TO CONTINUE DSCNT. AT APPROX FL320 ZDV TOLD US TO CLB TO FL330 AND TURN L APPROX 30 DEGS L OF COURSE. WE COMPLIED. THE ONLY REASON I CAN THINK OF FOR HAVING SELECTED DIFFERENT ALT FOR THE FMS AND MODE CTL PANEL IS A NOTE COMMONLY PLACED ON OUR ATL-DCA FLT PLANS ADVISING US ZDC ROUTING REQUIRES XING JAXSN AT OR BELOW FL310. WE ALSO WERE ON THE FINAL DAY OF A 4 DAY TRIP HAVING BEGUN THE DAY WITH AN XA30 WAKE UP.