Why does PH go up when rezevoir gets low?

If the water in a rezevoir is low and the PPM are higher than the starting amount, then why would the PH go up? Shouldn't the PH go down since the ppm is higher. The ph seems to go lower when the origional ppm get higher. Any input? Anyone?

Higher concentration of chemicals sinking to the bottom? I have no idea but that would be guess....