"Chapters [not descended from Guilliman's geneseed] are disciples who owe their genetic inheritance to another Primarch, but follow the Codex Astartes as keenly as their divergent heritage allows. While primarily composed of successor Chapters, this group also includes several Chapters of the First Founding - notably the Imperial Fists, White Scars and the Raven Guard. These chapters can never be Ultramarines, for their gene-seed is not that of Roboute Guilliman. Nevertheless, they will ever aspire to the standards and teachings of the great Primarch.
[Chapters who do not emulate the Ultramarines] are aberrants; chapters who, through quirk of gene-seed, mutation or stubbornness, eschew the Codex Astartes in favor of other structural and combat doctrines. Some, such as the Blood Angels and their successors, strive to be worthy of Guilliman's legacy, but their recalcitrant gene-seed drives them ever further from it. Others, such as the Space Wolves and the Black Templars, remain stubbornly independent, looking to their own founder's ways of war and caring little of how they fare in the eyes of others. These aberrant Chapters were always few in number and their presence diminishes with each passing decade, for their gene-seed is no longer the source of fresh Chapters." - Matt Ward, Ultramarines player, codex writer (unfortunately). (Also note that the Black Templars actually outnumber the Ultramarines at a ratio of at least 6:1)