Hello Ramon,
The power meter measures average power. However, OSA measure power spectral density (PSD). So, the noise bins have PSD data, which needs to be multiplied by the noise bin bandwidth and averaged out over the total bandwidth to get same power measured by the power meter. The noise bandwidth parameter can be read from the attached image.
Please try the calculation, if you have not manage to get correct answer, please contact me at ahmad.atieh@optiwave.com for further discussions.
Regards,
Ahmad