PvP Skill "Weapon Throw"

Hi, I was PvPing recently and bought the skill "Weapon Throw". The description claims to have a 20% of inflicting Heavy, and yet, repeated use saw Heavy inflicted each and every time. Also, the last level of Weapon Throw claims to increase a so-far unmentioned "potency" increase. After checking the Japanese description,

対象に20%ヘヴィを付与する

My suspicions were confirmed. The description should be "Inflicts Heavy with 20% potency". Or something to that effect, anyway. The chance is 100%, the Heavy slows you by 20%.

Same issue is present in Shadow Flare's description.