I don't account for the grain absorption or losses but there is a difference between the water profile of the mash and the sparge. During the mash the grain will affect the pH of the strike water and that water will be adjusted to account for this. During the sparge, the water should be adjusted as well so the pH is in the proper range but the adjustment might be different to get the desired level.
I use Bru'n water and note that it allows for both mash and sparge adjustments and typically they are slightly different in what needs to be added.
Take my comments lightly as this is my basic understanding with a lot of learning to come, perhaps Martin or someone with a greater understanding can chime in:)