they are, but they're noted to have sucessor chapters. that's the thing, not ALL space Marine chapters are DIRECTLY descended from a first founding chapter. Thus "The Emperor's Big Shoulders" chapter, might be a nsucessor chapter from the "Emperor's oversized codpeices" whom are sucessors of "the emperor's big helmets" whom are sucessors of the Crimson Fists, whom, as we know, are sucessors of the Imperial Fists.
it all depends whose geneseed was directly used in your chapters creation.