ERRONEOUS WT AND BAL. EXTRA FLT ATTENDANTS CAME ON BOARD; BUT NOT LISTED ON THE WT AND BAL.

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RPTR WAS ADMINISTERING AN INITIAL LINE CHK TO A NEW CAPT AND WAS OCCUPYING THE R SEAT/DOING NORMAL COPLT DUTIES. FAA OPS INSPECTOR WAS OCCUPYING THE JUMP SEAT TO OBSERVE. WE HAD MADE A QUICK TURN AT ORD AND WHILE I WAS AWAY FROM THE ACFT WE HAD 2 EXTRA FLT ATTENDANTS BOARD TO WORK THE FLT. BY THE TIME I RETURNED TO THE ACFT THE PAX WERE ALREADY BOARDING AND I CONTINUED WITH MY DUTIES UNAWARE OF THE 2 EXTRA FLT ATTENDANTS. AFTER WE PUSHED FROM THE GATE AND BEGAN OUR TAXI I RECEIVED (VIA ACARS) OUR WT DATA AND CHKED IT FOR ACCURACY. IT LOOKED OK; EXCEPT FOR THE FACT THAT OUR FAA OBSERVER SHOULD HAVE BEEN LISTED AS AN EXTRA CREWMEMBER; AND HE WASN'T. SINCE I KNEW THAT HE HAD CHKED IN WITH THE GATE AGENT BEFORE THE FLT I HAD NO REASON TO BELIEVE THAT HE HAD NOT BEEN INCLUDED IN THE GATE AGENT'S TOTAL PAX COUNT; I SIMPLY ASSUMED THAT HE WAS LISTED AS A 'PAX' INSTEAD OF AN 'EXTRA CREWMEMBER.' WHAT I DON'T KNOW; HOWEVER; WAS THAT PAX WTS ARE FIGURED AT A DIFFERENT VALUE THAN EXTRA CREWMEMBER WTS SO HE REALLY SHOULD HAVE BEEN LISTED AS AN EXTRA CREWMEMBER. SO; CREW COUNT SHOWED 5 (NORMAL = 2 COCKPIT PLUS 3 FLT ATTENDANTS) WHEN IN FACT IT SHOULD HAVE SHOWED 8 (2 COCKPIT PLUS 1 EXTRA CREWMEMBER PLUS 5 FLT ATTENDANTS)! IT WAS NOT UNTIL WE WERE AIRBORNE AND AT CRUISE THAT THE FAA INSPECTOR BROUGHT TO MY ATTN THAT THE CREW COUNT WAS WRONG!! HERE'S AN EXCELLENT EXAMPLE OF HOW COCKPIT RESOURCE MGMNT EXTENDS TO ALL CONCERNED INCLUDING FAA INSPECTORS AND EXTRA FLT ATTENDANTS...EVEN THE NEW CAPT. IF ONLY SOMEONE HAD INFORMED ME OF THE FLT ATTENDANTS THIS PROB WOULD HAVE BEEN PREVENTED. ALSO; HERE'S AN EXCELLENT EXAMPLE FOR ME OF WHY I SHOULD LOOK FOR UNDERSTANDING WHY WHEN I READ OUR COMPANY POLICY MANUAL - NOW IT MAKES SENSE - AS EXTRA CREWMEMBER MUST BE LISTED AS SUCH BECAUSE HE IS COUNTED AS A DIFFERENT WT!