The water quality varies from season to season as a result of rainfall and the source of the water prior to treatment.
As the aquifers fill and empty, the amount of calcium and magnesium held in solution will vary. The speed of the water percolating from the surface will affect the hardness and this can change when water companies need to share water from one area to another, on occasion where one is short. So depending on where you are in the country, the water quality can vary quite considerably.
However, some areas are very stable. The quality of ground water will vary according to the depth from which it is drawn. If you have a period of heavy rain, the aquifers fill up and dilute the hardness and when there is less water, the hardness has concentrated because the water is in the aquifers for longer and is therefore capable of dissolving more limestone.