Nagelring is not working correctly. When you use Nagelring on your hero and Fallen Lunatics explode damage calculation takes into account your +% of physical damage. But when you use Nagelring on your follower and Fallen Lunatics expolde damage calculation does not take into account your follower’s +% of physical damage. For example using Defender of the Westmarch on your follower calculates everything correctly (Wolf uses +% physical damage on your Follower). So this bug is only when using Nagelring on Follower.