1) Its really bad to think of any particular geneseed as those specifically engineered for one purpose. Because otherwise that dumbs down space marine actions to purely determined by genetics and not their nuture. For instance, the Iron Warriors and the Imperial Fists have geneseeds that give them almost identical character traits and savant skills. But between the two legions, only Imperial Fists have developed the legion wide culture of a near suicidal disregard of personal safety. But by your arguments, both legions ought to have near identical legion identities and combat doctrines
2) A lot of chapters differ greatly in doctrine compared to their parent chapter. And this has a lot to do with the nurture aspect of the individual chapter. For instance, the Death Scythes are an Ultramarine successor yet have chapter masters that never fought with the chapter. Ever. For any marine that flew the heraldry of the 13th's sequined omega, this doctrine would've sounded like blasphemy. However, the truth behind this is that the Death Scythes' chapter master is forced to sit on the shariax, a warp resonant xenotech artefact that drains the soul of whoever is sitting on it while guaranteeing that the Ghoul Stars "are not hit with a catastrophe". Should the Shariax not have someone sitting on it, the running theory is that whatever is sealed by the Shariax deep in the core of Occludes will awaken and Devastate the Ghoul Stars indefinitely. For the Ultramarines, the Death Scythes' very sacrifice of their own members is borderline heresy.