Help understanding ratios CBD / THC, throughout the distillation process

If folks are using biomass which according to COAs test at 15% CBD and .3% THC (for the sake of discussion); that is a ratio of about 50:1 CBD / THC. It’s not my field of expertise but as far as I understand ratios should remain consistent throughout the distillation process. So why is it that on full spectrum distillate the ratios I’m seeing are more in the area of 20(ish): 1? I’m trying to educate myself a bit so thanks in advance.