Why does the left panel with the header "Artist" sometimes show album artist?

[Dalinar]

I have a bunch of Audiobooks and in the left pane it is sorted by "Artist". It works for most of the files.. I mean usually I will see the artist (i.e. the value from the Artist tag). There is no grouping.

but with some books, it will show the "Album Artist" tag instead, even though the Artist tag also has a value (which is different than in Album Artist)

Does anyone know why this is happening and how to fix it? (to show the artist tag for every album I mean)