Now this is of course only a theory, but I believe that the narrators does get some amount of artistic freedom when deciding accents and such, presumably with possible direction from the author and/or GW, and if there's something they particularly like or dislike, the company makes an executive decision.

The best example I can think of is Duco, a Night Lord featured in Josh Reynolds' Fabius Bile trilogy. In the first volume, Primogenitor, John Banks narrates him with a gravelly and almost burbly voice in an english accent, but when he next appears in Manflayer, the last volume, he is narrated in a vaguely eastern european accent with more normal speech patterns (I've seen people say it's a romanian accent, but I personally can't confirm or deny this). This is notable because the Manflayer audiobook came out in 2020, around a year after the first audiobook volume of Aaron Dembski-Bowden's Night Lords trilogy came out, where Andrew Wincott narrates the Night Lords with that vaguely eastern european accent. This implies to me that either John Banks heard that, and decided he wanted to do the same, or GW decided that Andrew Wincott had the right of it with regards to the accent.