FFT and Third octave discrepancy - Sound and Vibration

[alanesco]

Hello all, I need some help with a problem that Im having. Im currently analyzing a calibration signal and when I perform the FFT signal analysis I find a peak with amplitud 93.1dB @998,66Hz and 2 peak sides of aprox. 87dB (marked in the picture attached). Then i performed a Third octave analysis and I found that the Band related to 1000Hz has an amplitude of 93.1dB. My question is: just the sum of these 3 bins  gives me an amplitude of aprox 99dB, why the amplitude of the band is 93.1dB? What Im misunderstanding? Thank you so much for your help.

Regards,

Alan

[MCOTO]

These posts provide some information about third octave analysis.

http://forums.ni.com/t5/LabVIEW/Amplitude-Spectrum-and-1-3-Octave-Analysis/td-p/2289924

http://www.ni.com/pdf/manuals/320952a.pdf

Regards,

MCOTO