I have no albums in the for example "children's", "blues" or "country" genres. So why do these genres show up here under "Genres by number of albums" ?
Roon’s Genre Browser is fueled by all kind of source data, not just your library’s albums. Artist genres (who may also stem from works not in your library) and I’m not talking about album artists only here but all credited artists. You can even find Radio Stations here. While it is possible using Import Settings and the Genre editor to enforce, to some degree, the genres of imported (file based) albums, you will reach the limits here when streaming services come into play and for all other data points with genres attached (artists, compositions (I think), radio stations) you have no other chance as to find and correct each and any instance of those data points and manually edit the genres there. Did you look into one of the unwanted/unexpected genres and what did you find in there?
Thanks for writing in and great question - @BlackJack is correct in the above reply. Roon pulls genre metadata from external sources, which often include hierarchical relationships — for example: “Rockabilly” might be nested under “Country”,“Delta Blues” under “Blues” and “Lullabies” under “Children’s”. On top of this, Roon supports multi-genre tagging. A single album may be tagged under several genres, including ones you might not expect.