Xbox music pass doesn't cover all songs?

I am doing the 30 day trial of the Xbox music pass because I figured "why not?" I am coming from using Spotify previously. Am I missing something, or is there content that is not included with the pass? I am going to listen to an album and the only option is to buy or preview it. Then I'll jump to another album and it will let me play the entire thing. Seems weird to me since both were included on Spotify as part of the service. Anyone else seeing this / know the answer?