Phasezerker Mod Cooldown 1/4 of Listed Bonus

Phasezerker lists 10% cooldown per Rush stack. 30s Phasecast and 25 rush (decaying to 17 as time passes) should yield a 10s cooldown from +200% cooldown rate… Instead it gives ~20s. This is closer to 50%. I have tried reloading, respecing, switching characters, etc. to clear problems with equipping mods. It does the same for all action skills, rush stack maxes, and with/without Avatar.