rear disc from grand cherokee on a 8 3/4

Im still trying to wrap my head around the difference between the 8 1/4 & the 8 3/4 at the axle flange that allows the jeep brakes to be used on one with no difficulty and not the other.
I have a 8 3/4 to measure I could use some info on the 8 1/4.
Is it that the 8 1/4 axles stick out further than the other as the factory set them up?
If I can get the info I will try to engineer an answer for all
Andrew