The relation between gravity and pH is such that the alkalinity of the water plays a big role. It provides the counterweight to the acidity of the wort. The more extract you have in water (i.e. the more gravity) the more it will determine the pH. With alkalinine sparge water the bicarbonate of the water will aims at raising the pH. The more there is and the lower the extract content the higher the pH will rise. This also means that in the absence of alkalinity, or very low alkalinity, the pH will change only little as the wort gets more dilute. I have seen the pH rise as much as 0.4-0.5 points in batch sparging. But this only happened with alkaline sparge water and when I either did 2 sparge batches or made a small beer.