Well because Deadeye raising other Death Knights might not be Bolvar’s will. Not saying Bolvar would necessarily be against it, but he might be utterly uninvolved with the situation.
To me it appears Deadaxe has no interaction with the Lich King outside the fact the Ebon Blade as a whole is working with him. His personal ability or actions need not necessarily have Bolvar’s consent to be enacted.
Of interest, there are the Onslaught Death Knights that are probably made by Mal’ganis. Given Balnazzar shows the ability to raise undead and these Death Knights are aligned with the Scarlet Onslaught under Mal’ganis working against the Lich King.