Grievous Woudns

Why does executioners and mortal reminder apply grievous for all physical damage to all health ranges, but morellos applies to only those below 35% who take magic damage, and only on fully completed item? It seems clear that one costs far less (900 compared to 2100) and has far greater reach of application since its application can be more consistent (auto attacks) as well as physical damage abilities. The duration seems a bit irrelevant as executioners and mortal reminder can be reapplied by auto attacks.