The mean(zonal^2+meridional^2) is not equivalent to (mean(zonal)^2+mean(meridional)^2). The same principle applies to the kriging method, the objective interpolation used to estimate the mean wind fields, so components and module are computed separetly. Otherwise, the wind module would be generally underestimated.
The wind vectors are retrieved from ERS-2 scatterometer. The failure of some gyroscopes onboard ERS-2 platform caused the satellite to run in a coarse attitude control mode. As a consequence, scatterometer data products are not calibrated anymore and not distributed to users. A re-design of the scatterometer data processor by ESA shall be performed to restore the initial data quality. This reprocessing is not planned yet.