Bottom line, the water sticks to the grain but the salts and sugars move from bound to free water.
Your process is pretty close, you're just low by whatever percentage of sugar/minerals comes from your sparge. Probably around 10%. None of these flavor minerals is that touchy unless you're running right on the ragged edge of being too much. And then I doubt its a matter of 300ppm SO4 being OK and 330ppm being awful.