Comment 3 for bug 496129

This is the so-called "fixed slot allocation". The first slot is reserved for "synchronous" notifications, which I believe means they respond to user feedback. E.G., you can adjust the volume while the notification is still on the screen and the volume meter updates.

The distinction is meaningless to the user, in my opinion. Considering that having one notification of both types is probably not as common as just one, the behavior just comes across as inconsistent and broken.

The fix is easy enough, just change the default allocation to dynamic.