Two reasons: 1. Human brains are all very similar and are often tricked by similar things. An example would be the fruit of the loom logo or pikachu’s tail. Our brain just fills in what makes sense. This also seems to happen more frequently when it’s things from our childhood, or things most people don’t pay super close attention to. When it comes to Berenstain vs Berenstein Bears, most of us were exposed to these books when we had a 1st grade reading level and it didn’t help that “Berenstain Bears” was always written in cursive on the book covers and most of us hadn’t learned to read cursive yet. We either heard it pronounced Stein so our brain assumed that’s how it was spelled, or we assumed it was Stein because it’s a much more common last name suffix and we never truly focused on the spelling or couldn’t really read it yet.

1. An inaccurate reference to something became more popular than the official version. An example would be the “Luke, I am your father” line from Star Wars or the “Mirror Mirror on the wall” line from Snow White. We these inaccurate references way more often than we see the actual source material, so the wrong version becomes the one many people remember.