It would appear that Cyanide has more room for manoeuvre when it comes to the College rosters (that is, stuff which Games Workshop hasn't updated in an official capacity for ages) than with the BB rosters which the College rosters are based on (thus, the alleged inability to remove that pesky Deathroller from the *Dorf* roster).
Yes, you're supposed to find the above convincing
Anyway, having Chaos as the main race of the Grey College is hardly what I'd call a capital offence. Turning Humans into the main race of the Bright College was a good decision IMO (Dorfs would be too prevalent otherwise), and this makes the changes in the Grey roster somewhat understandable (if Humans were the main race of two out of five available Colleges things would again look quite same-y).