The blanket statement "GH will drop over time" is NOT true for everyone. Each tank and situation differ. In some situations, GH will increase over time. It depends on the substrate, what you dose, how often you dose, and if you are replacing evaporated water with tap water.
For example, if you don't have anything in the tank that has an effect on lowering GH (such as peat), and you top off the tank with tap water over a two week period, and that tap water has some level of calcium in it, GH will definitely rise over that two week period. (How much would depend on how much water is evaporating and being replaced, and the level of GH in the tap water.)