Sometime the Wither's skull projectile fail to deal damage when they hit a mob or a player.

In videos where the Wither is fighting mob like Iron Golem or Warden, sometime when it shoot a Wither skull at them, it doesn't deal any damage. And if you look closely, the Wither skull doesn't actually hit the ground it hits the mob directly. If it had hit the ground, it would have created a pit.

​In the video the game's difficulty is normal and I use command block to clear the wither effect.