Originally Posted by DeathJester921
"The Imperial Fists' naval might was the greatest of any of the Legiones Astartes..... Ultramarines could not rival such strength alone."
From that quote you pulled from Extermination. I don't see anywhere that it says the Ultramarines have one of the largest fleets. Merely that they could not rival its strength. Considering the Ultramarines grasp of all things tactics, I can see them being placed among the best of the legions in terms of naval warfare with just a mid-sized fleet.
Unless you misquoted it, there is no contradiction here.
Because the Betrayal has explicitly stated Sons of Horus have one of the strongest fleet among the Legions and the quote is rather obviously implying Ultramarines navel strength is on a par with them. Moreover nuance of citation is clearly expressing Ultramarines have one of the largest fleets.