GarageBand - Missing Quit Button

Unlike every other application on my Mac, the red "close" button on GarageBand doesn't close the app, rather it closes the main screen and calls up the launch screen to start another project. On the launch screen, the close button is greyed out.

I'm still able to close the application by right clicking the GarageBand icon in the running applications tray and clicking on "Quit," but this seems like an odd way to have to close the program.

Any ideas of why GarageBand is not following the standard close app convention?