Kench W Projectile damage lower than on tooltip [gameplay]

Spitting a minion at enemies has always felt like it did practically no damage to me. Looked at numbers in game, and it was actually lower than it's supposed to be.

In game, with no ap or magic pen Rank 1: Base 100 magic dmg Enemy thresh had bond of stone mastery and enough mr that it said 26% dmg reduction in mouse-over text. Thresh took only about 50-60 damage from this ability, which is too little for the mr and bond of stone dmg reduction he had. Not sure if this is a tooltip error or a dmg error.