Music has, through various court decisions through the years, gained a particularly arcane and arbitrary set of rules on what parts of a piece are copyrightable, and to what extent. It's a mess, as some parts are overly-protected, like the layout of the music on the page (unlike the typesetting of a book, or the font used!), and some under-protected in comparison, like the rhythm of a song compared to it's melody (which has a particularly arbitrary rule of 7 notes long). Though, to answer the question of the last part, if that version was copyrightable, it depends what book the performers are using! Though again, it could be that the essence of the performance is not considered to be a violation, even if photocopying the book would be, depending on what form any differences take.

If you're concluding copyright is impossibly complicated, vague, and confusing and the only reason the system works at all is because hashing out the details in court is either a nuclear option between two large companies, or enforcement on a large scale against small actors is basically impossible (but will unpredictably come down like a ton of bricks on some unlucky individuals), then you'd be right!