I thought it was possible for a weapon to have a magical bonus LOWER than the material/craftmanship bonus. If, for instance, a weapon is +10 normal and +5 magical, you'd usually use it as a+10 normal weapon, but when fighting large/superlarge creatures, or creatures that require magical weapons to hit, you'd use it as a +5 magic weapon. I'm pretty sure I've seen such weapons, but don't recall where.