Oo! I know this one.
(and there are no dumb questions!)
When water evaporates out of the tank...just the water goes....not the minerals (or other chemicals) that are dissolved in the water. So if you're just topping of your tank, you don't want to add any additional stuff to the water (since the stuff you initally added is still in there).
When you do a water change, you're physically removing the water and everything dissolved in it, so you need to add the RO
Right into your water to accomidate for what you took out.