Game creates a new player or parallel universe after joining a realm

I have no clue how this works, but whenever you join a world after you join a realm, it goes on some other "Player" or some parallel account. I have no clue how this works but I still affect the world and have an inventory. After you leave the world again, whenever you join back you are on your main character. It's not actually another account or player I just have no way to describe this.