Garageband probably draws its own window. I'll try to explain:
You see, when creating an app, you can use a stock window which the OS provides, and automatically comes with all the functionality of a window, or you can "draw" your own custom window, in which case you need to code everything yourself, and provide the button images.
Garageband's window is different from the "stock windows": it has that wooden left and right borders, which require the buttons to be pushed to the right. That means they probably draw their own window, which allows this level of customization. The garageband app has bitmaps for the buttons and draws them itself, instead of "asking the OS to do it".
Other apps simply call for the "stock window". If the OS changes this "template", all the windows that rely on it automatically change.
Other apps might use custom windows too, but embed bitmaps for different visual versions of OSes. Nevertheless, this is a lot of work for minor details and not all developers do it.
I guess this will be addressed in a future version of Garageband. It's not a big deal as you say, so perhaps not soon, but yeah, I would also prefer the app to be more consistent to the OS.
Hope it helps!