Sorry, really late on seeing this!
Numu's artist and release information comes from MusicBrainz.org, which is basically like wikipedia for music. Any info added to MusicBrainz will eventually end up in Numu (can take up to 3 days).
​
If you find that an artist is matched incorrectly to something in MusicBrainz that's random, let me know and I can fix it pretty easily.
Release and artist data in Numu comes from MusicBrainz.org, which is essentially "wikipedia but just for music", so anyone can sign up and start adding artists and release groups/releases to it. After adding data to MusicBrainz it can take up to 3 days for the data to appear in Numu.
I know from feedback from other users that some relatively niche genres aren't reflected very well in MusicBrainz and could definitely use more contributors, for example drum'n'bass is another genre that lacks representation.
There's also a chance that an artist you listen to has been matched to a similarly named but incorrect artist on MB, so if you find an artist on MB and there are releases on MB that don't appear in Numu, that could be the culprit. In that case, just let me know and I can fix that incorrect match.