Base dps DOES include additional fire damage, etc. Look at a legendary item where the added damage is variable. Your dps math is correct, though. Your logic is wrong. Or look at the same magic base item with different damage properties. One that adds 2-4 damage will display a lower dps than one with 2-5 damage. Damage type (arcane, poison, etc) is irrelevant. The dps shown on your screen is based on weapon damage, primary stat, crit chance and damage, and added damage from rings/amulets (which is added to base weapon damage, which is why it is so good).