It's true, they are very different, but I always thought that the idea of that was to highlight the difference between Bellatrix and Narcissa. Although they both believe in the pureblood supremacy (Bellatrix more than Narcissa though!) Bellatrix is way more dark and cruel, whereas Narcissa cares about her family and never seems to be nasty for the sake of it. She also redeems herself at the end, by lying to Voldemort.