I'm going out on a limb here and saying it isn't as much the caps moving as it is the deflection of the pinion shaft and the case which supports the pinion against the ring gear.

In most any gearing system it's the smaller gear, called the pinion, that's most highly stressed.

The 9" has a support bearing or bushing on the other side of the pinion and that maintains the alignment much better. It doesn't seem like much but it makes quite a difference.

One disadvantage of the support is it moves the pinion lower on the ring gear, which eats more power by friction. The support also robs space for a proper limited slip differential. 9" limited slips are kind of one-sided. Of course, spools don't mind at all.

All in all, the advantages outweigh the disadvantages when we move into higher torque loads.

I see that Dr.Diff has a nodular 8 3/4 case for sale. Nodular iron is 50% stiffer than gray cast iron. This will lessen the amount of deflection for a given load. The nodular case should be the first purchase if you're building a stout 8 3/4.

R.