It seems that the Ultramarines in M41 have this habit of accusing each other of heresy. At the end of the story our Ultramarine Hero (much like Uriel Ventris) is accused of heresy by one of his battle brothers. Often because of the same thing: Not adhering to the Codex Astartes. I could imagine that an Ultramarine that joins Deathwatch is either: Not going to take the Adeptus Astartes too seroiusly, or is going to be in constant conflict with the other chapters. Think about a Spacewolf having to listen to the Codex bla, bla
I recommend the game though, its fully grim & dark, the way a desperate battle on a forge world should be. The orcs are classic too.