It was me on the old forum that started off the discussion about adjusting the calibration for the WMR968 anemometer.
We did a load of testing here between assorted anemometer makes and came to the conclusion that while the OS device measures very accurately in a constant flow (checked by attaching it to a pole sticking out of the car roof and then driving it at a constant speed on a still day and using a GPS speedo to check it) the problem occurs on dynamically varying days due to the difference in sampling rate between the OS device and a Davis for example. The longer sampling period naturally smooths out the short gusts so the anemometer appears to record lower than the much faster sampling Davis. The 1.25 correction factor simply brings its apparent measurement back into line with the Davis. Its only advisable to use this in a domestic environment where the surrounding houses tend to make the wind speed very lumpy and turbulent. If you were installing it on a cliff top, for example, I would not recommend using the correction factor.
The factor needs to be applied to both average speed and gust to be consistent.