Why in the formula for normalized Phase Noise (FOM) the contribution of PDF is calculated with 10*log10(PDF) and not 20*Log10(PDF)? In other wordl what is the explanation and the origin of this term?

Why in the formula for normalized Phase Noise (FOM) the contribution of PDF is calculated with 10*log10(PDF) and not 20*Log10(PDF)? In other wordl what is the explanation and the origin of this term?