Couple of reasons for this. First, versus a traditional mash tun, there is a lot more drainage area from the grains. Next, the compression of grains caused by lifting the bag releases some of the free water which is contained between the grain particles. Third, many people squeeze the bag and release more of the free water from the grains within.
I use three different brewing set ups: two of them BIAB and the third a cooler mash tun. They each have a different grain absorption rating based upon the amount of grain and my subsequent handling practices for each. It would be better, IMHO, to tie in the grain absorption rate to the equipment profile instead of having it as a globally set variable (hint, hint Brad).