Splash Potion of Instant Damage takes affects, no damage taken

Whenever you throw a splash potion of instant damage at a mob, it will act like you threw a snowball or egg at it unless it's health is low enough it will make it's death sound, drop it's drops, but not actually die. I've done it on several different mobs, and I'm not sure if it's the same for regular potions or tipped arrows.