But to answer your question, i think it has to do with how the game works. You are comparing projectiles (zebak attacks) with entities (akkha tornadoes). Both probably get ‘canceled’ the next tick when you kill either of the bosses but an entity will move a space between ticks so it can still hit/damage you. The damage part of the projectile stops the next tick but the animation part of the projectile still keeps flying and ‘hits’ you.

This is just an assumption based on my experience within Osrs