I've been wondering about this as well. Does anyone know if it's intentional or a bug?
There's no rule that says the naming system is required to be consistent, but it gets to be confusing. It's odd that Dark Radiance at the Arkanian tier does internal tech damage, but does internal force damage at every other tier. And Dread Guard Dark Radiance does tech damage if you have one from before 2.0, but any bought after 2.0 do force damage.