I agree they are not equivilent. In adding water from evaporation your basicly pardon the pun. 'watering down' the concentration of salts, trace elements and other DOC
's in the water. As water evaporates these levels get more concentrated in the remaning water. The change is fairly suttle since you should be doing daily or bi daily topoffs. The saltcreep you see is whats left behind when saltwater evaporates.
When you do a water change your physically tanking water out of the system and adding clean water back to the system. Your not only diluting whats in the system but your also removing some of the DOC
's and impurites as well as adding back into the system some trace elements.