Why HV polarization have best correlation with AGB (Aboveground biomass)?

I have used L band HH and HV polarised data to correlated my field estimated AGB. It shows better correlation in the HV band. I just want to know what is the actual reason behind it?

cross-polarization is more sensitive to volume scattering.
It is nicely explained in this document: SAR Handbook: Comprehensive Methodologies for Forest Monitoring and Biomass Estimation (nicely explained in chapter 3.3)

The same was found in this study: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2009GL040692 (Figure 2)