Total water needed and volume into MT are not the same

Brewing a 20 pound AG beer using a double batch sparge (put half water in MT, let sit 60 min, drain, put 2nd half water in, let sit 30 min, drain) process.  Brew Smith lists total water needed as 9.1 gal but shows water additions as 5.87 and 5.62 for a total of 11.5 gal.  Brewing to day so will go with 9 gal and read comments as they come in.

Boil volume is listed at 6.4 gal for a 5 gal batch which seems right. Using a 10 gal Igloo cooler and an 8 gal brew pot. Using 11.5 gal means 5 gal would have to be lost to grain absorption. Using 9 gal would mean losing 2.5 to absorption. Still high I think but excess can be boiled off if needed.

Any comments on differences between total water needed and water added to MT?

Does your equipment profile have a large "lauter tun deadspace"?  Or do you have a large trub-loss in your kettle? 

these are the two things that I would expect to cause what you are seeing.