It doesn't have to be a bug. If it's not, it basically means that you get 15 skillpoints per 10 levels, instead of the 12 you get in normal game modes. That's just 3 extra... So if you end up with a level 30 character, you'll still only have 9 more skills than you normally would.
The bonuspoints switch from character to character to make the game more interesting. First 5 levels in which you can make your DK really strong, followed by 5 levels in which you can make your own character really strong, etc...
Why not?