if you are trying to match other brewing software that might quote grain absorption in other units (e.g. L/kg) be aware that 0.96 is not some magic constant 
Your look into this is an excellent point that needs just a bit of context. I think it's very important to note that BeerSmith allows adjustment of absorption. Other software may not.
Absorption isn't absolute when thinking from a brewer's perspective. There is, of course, the amount of absorption that a single grit of pale barley will take up. But, there are also many other considerations.
First, the weight of a fluid ounce will be slightly less (around 3%) at mash temperature (60C) than cold (4C). The size of the grit will effect how much water it'll release.
We can then get into the amount of absorption from specialty barley and other grains. There's also the water that is inevitably trapped between grains ans mash geometry comes into play. Finally, there's the practical idea of how long do you want to wait for all free water to finish draining (it's a pretty long time).
I think the takeaway is that determining the actual absorption is important for those who need to conserve every drop. However, for most, the 0.96 is a pretty good number.
I have found the number changes depending on the brewhouse and the grain crush. I try to dial my brewery in to get maximum extraction (90+%) and tighten the water used. My absorption number is 0.7020.
