It is based on this work: https://www.ecmwf.int/sites/default/files/elibrary/2003/9861-cmod5-improved-geophysical-model-function-ers-c-band-scatterometry.pdf
I’m not sure about your issue but there are some discussions on this tool which might give you some hints:
- Wind Field estimator bad performance for strong winds (S1A data)
- Wind field estimate results different VV and VH
- Wind Field Retrieval Result Report - Same location but different wind speed estimates
- Wind field estimation for S1-SLC
- Wind speed from Sentinel-1 images
- Wind field estimation results are incorrect