You know it's not strictly auto completing sentences that previously existed, right? It's using words that it anticipates should follow others. I've had it suggest code libraries that don't exist, and you'll hear about people going to the library to ask for books that haven't been written but supposedly by real authors, and it sounds like something they would write.

Tab music notation is super common, and although it wasn't supported by this particular service before, you could see where it might be the sort of request people make, and so chatgpt combined the two.