Why can't I agree post boil volume?

Beersmith 2.0.57 comes up with a post boil volume of 14.04L.

When I do the calculation manually, using the volumes supplied in the BS recipe I get;

Sparge volume ............................11.81
Water availalable from mash.......  4.92
                                                    16.73

less;
boil off...................1.30
cooling loss...........0.56.................1.86
                                                  14.87 (a difference of 0.83L - there are no other provisions in the recipe that could account for this difference in volume)

Why are the post boil volumes different?

grain absorption?

No its not grain absorption!. Beersmith has already factored that into its calculations, as 2.2L.

Let me ask a different question.
Are your manual calculations of post boil volumes always around the mark compared with Beersmith calculations?

This issue is obviously pretty important to outcomes so if you can suggest a reason for the anomoly I would be grateful.

Thanks

Post boil is before cooling loss.

My guess is your "sparge volume" and "water available from mash" are including something that shouldn't be there. Since you aren't using entries that are in BeerSmith nobody can possibly know except you. We can't check your figures because you don't tell us what they are.

If I take my strike water plus my sparge water minus the grain absorption I get real close to my boil volume (.03 gallon off). If I take my boil volume minus my boil off I get my post boil exactly.