Joe Hills is a law of the universe. In the same vein as gravity or magnetism or entropy or the fact that when you die you don’t come back to life. Except Joe Hills, (or The Joe Hills Difference) is the law that says that all other laws can be broken at random times. An example of this is Cleo! He’s the reason Cleo is not only undead, but (mostly) peaceful! Occasionally, Joe will randomly decide to mess with his siblings, pop somewhere random, and make it so they are defied. Joe is no longer invited to thanksgiving. He still shows up though.
Joe decided to be human because he liked books. He really likes books, he loves books! So he decided that to truly enjoy books he should get a physical form! Due to Joe being, well, Joe, his body very rarely obeys his siblings. It is not unusual to find Joe floating around. In fact, it is kinda expected at this point.
Joe and Gravity are particularly at odds, but that’s because Gravity used to be the baby of the family before Joe came along. So Joe defies Her the most.