Matt.Kingsley wrote:That quote talks about entire chapters falling, which is admittedly rare. It does not talk about the entire number of marines that have fallen (ranging from individual marines, squads and even companies in additional to the aforementioned ~50 chapters)
Which comprise the extra 50 or so extra Warbands that we can safely assume exist. Also, something you may forget: If a part of a Chapter goes rogue, that puts the entire Chapter into the suspicious eyes of the Inquisition. In light of this blight on their honor, most Chapters will hunt down and destroy any of their number who turn traitor (see: Wolf Brothers).