It's all comparative - by the standards of its AV receiver competitors at the £1000 mark (when it cost that much), the Onkyo 875 doesn't do too badly for music (and blasts them all away for movie performance).
The Onkyo 905, however, is in a price bracket where it's up against higher-end receivers (Marantz etc) that are designed to be far more of a music-led prospect. As we comparatively test everything, that means the bigger Onk is going to lose out on that front. But, again, it's by far the better all-rounder of all the options in its field.
Again, it's all down to what you want/expect your receiver to do. For many people, both Onkyos will be more than musical enough. For others, they'lll forgo the superior movie performance and feature spec for a more musical amp.