I recently bought a mill and decided to start double crushing to improve efficiency. Since doing this, my boil volumes are greater than expected. I am very accurate on measuring out malt weights and water volumes, and until now, I could hit very close to my expected boil volume. I use 0.5 quarts (0.125 gallons) of water absorbed per pound of grain. I only factor in 1 quart of dead space in my cooler MLT. The last beers have been bigger ones, so I'm not sure if that's a factor. I have a difficult time getting much over 70% boil efficiency when I make these big beers which is why I decided to do a double crush. The last beer I brewed I got 79% on a 15lb grain bill for a 5.5 gallon batch, so I did see an improvement, but I was shooting for 6.8 gallons boil volume and got 7.5 gallons. The only thing I can think of is that there is less water absorption with a final crush, or maybe it initially absorbs that much water but then begins to release it at the end of conversion? I know this sounds crazy, but if you have smaller grain pieces and have good conversion of starch to sugar, maybe the spent grain can't hold all that water anymore? Or is there something else much more obvious going on here?