B737-300 FLT CREW WAS GIVEN INCORRECT WEIGHT PRIOR TO TKOF. CORRECT INFO WAS RELAYED TO CREW AFTER AIRBORNE.

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ALL PREFLT AND LOADING WENT NORMALLY. THE OPS AGENT HANDED IN THE LOAD SHEET. THE FO STARTED HIS CALCULATIONS ON THE OPC AND I DID A REVIEW OF THE LOAD SHEET. THE MATH ADDED UP AND THE GALS ADDED WERE CORRECT. THE FLT ATTENDANT HANDED IN THE NAPKIN AND THE PAX COUNT MATCHED AT 98. THAT GAVE US A REDUCED N1 OF 91.3 (APPROX). TKOF AND INITIAL CLB WAS NORMAL; THOUGH WE BOTH COMMENTED THAT THE OLD B-300 WAS NOT IMPRESSING US WITH ITS PERFORMANCE. AT 13000 FT; N OF FMG; RNO CALLED US TO REVISE OUR ZFW AND TKOF WT. THE REVISED NUMBERS WERE: ZFW 102344; TKOF WT 112946 VERSUS LOAD SHEET ZFW 96645; TKOF WT 107247. THE ACTUAL PAX COUNT WAS OFF BY 30; 128 VERSUS 98. I CALLED THE FLT ATTENDANT AND ASKED HOW MANY PAX WE REALLY HAD BACK THERE. SHE INSTANTLY KNEW WE HAD 128 AND THAT SHE HAD ACCIDENTALLY WRITTEN 98. I RECOMPUTED THE DATA IN THE OPC AND THE REVISED TKOF N1 WAS 93.8 (APPROX). WE UPDATED THE FMC WITH THE CORRECT WT; AND THE REMAINDER OF THE FLT WAS UNEVENTFUL. WHEN WE LANDED; I SPOKE WITH THE A FLT ATTENDANT AND EMPHASIZED SHE IS OUR SAFETY NET. SHE HAD COUNTED CORRECTLY; BUT HEARD 98 AND WROTE IT DOWN. THE OPS AGENT DID NOT ADD THE THROUGH COUNT. SHOULD THE COMPUTER NOT KNOW THERE ARE SOME THROUGH PAX?