you have to remark that moisture is just one factor contributing to backscatter intensity. It also depends on roughness and vegetation cover of the surfaces which, as I expect in your case, superimpose the variation of the moisture.
Were the images acquired at the same time as the field data was collected?
@johngan has outlined this topic quite nicely: Soil moisture identification -Sentinel
You have the advantage that you have measurements, but the ambiguity of backscatter intensity remains a problem.