Okay, it wasn't entirely created by someone different, since a couple of the pieces of music in GK3 originated in GK1. Still, I thought, despite the fact that they bafflingly have different last names, Jane Jensen and Robert Holmes were supposed to be married. Why did she ditch her husband for GK3? I wonder how he felt about it.
Actually the GK3 music was co-composed by David Henry. Robert Holmes did work on the game, and provided some of the new themes (interestingly the closing credits music for GK3 was a piece which Robert Holmes originally composed for a never completed game adaptation of Jane's novel Millennium Rising)
If you check the credits, you'll notice that the GK2 music, similarly, is co-credited. This time to Jay Usher. On GK1 Robert Holmes was the producer of the game, a role he didn't continue for the sequels as he was working on his own musical career. So it makes sense that his role in the production of the game music was shared with others who would be spending more time on the game itself.
Oh, and there's nothing "baffling" about married couples who have different last names. It's a very common practice.