I had never noticed it before, but you're right - not all tracks have both the artist name and track title showing up.
The XBox Music Style is making use of the NowPlayingList element for that section.
And that element has a child attribute called TrackArtistAndTitle which should be showing <artist> - <title> for all tracks, but it isn't for some reason.
Might be worth looking into by Steven if he comes across this.
In any case, you can replace that child element by something like this which should serve the same purpose:
<child id="Artist"/><child> - </child><child id="Title" width="50"/>