Pre boil volume discrepancy

In my recipe I am using an equipment profile that uses a pre boil volume of 31.58 L. The displayed fields in the recipe, however, shows the pre-boil volume to be sparge (20.88) + mash water (12.9) =total 33.78L.

Could you suggest or explain why there is a discrepancy of 2.2L.

Thanks.

I haven't done the math but I would assume grain absorption. You don't get all the water you put into the grain back out.

If you find that to be too far off there is a place to change the amount of absorption in the options/advanced tab.