A lot also depends on the dyno and the software used by it. As an example the dyno at Torque Performance where I go they have been working relentlessly on developing a software package that will compensate for the drive line losses from 2wd to 4wd.
We have put my GTR up on a number of occassions now in both 2wd and 4wd. It has produced simular numbers in both configurations withing 1%.
Dyno's that are unable to do that or houses that honestly have not time or can't be bothered usually go off a set percentage loss. From car to car it makes it difficult because each drive line is slightly different and the effiency of it make calculating percise numbers that much more difficult.