[DeathJester921]

I dont see any contradiction here, honestly. In both passages the Ultramarines fleet strength is still stated as being behind the Imperial Fists fleet strength.

What is the contradiction you're seeing?

 

[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.