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.
Bingo........The CASE is what flexes, allowing pinion to move away from ring gear. The cap has zero bearing on this. It pushes the cap off when all the tooth debris gets lodged in there and SOMETHING has to give, that's the cap. A better cap may not break, but something else will. I have seen 8.75s with GOOD caps still shell teeth. When it cant break the cap, I have seen them break the pinion clean off or split the case. You get all that junk lodged in between SOMETHING is going to give
