Look the 1TB has drastically faster storage already probably due to having parallelism (2) 512gb nand chips soldered on vs just a single 64/256/512
I’ve compared a 11” 512gb to a 11” 1TB model side by side loading internal storage to full on each model on the same backup. Then testing ram usage on each one.
You can deplete a 4GB ram iPad Pro and it starts draining memory from system services, and idle applications as it sucks every drop of the 3.7gb to your specific demanding application.
It is a old wives tale.
Whether the iPad Pro 1TB is slammed damn full with 956gb it doesn’t utilize any more memory than if it were fresh and formatted with no storage used at all.
Both devices are missing a certain amount of ram that may be allocated for SSD caching. That 4GB is really only 3.7GB. And that 6GB is really only 5.68GB. So the 1TB still has extra ram set aside for something while still having 1.9gb extra memory.
Unless you tested the 1tb with 4gb ram, you don’t know anything about its performance with less ram. The dram cache is there for speed, if it is depleted on a very large transfer (say more than a smaller ssd like 256GB could sustain) then the performance can drop dramatically. I don’t really want to go hunting for technical papers on ssds for you, but yes large high performance ssds need significantly more memory or they risk being degraded in performance.
So apple had a choice, either give all devices 6gb or just give the 1tb 6gb so it performed as well as the others. Is there ram left over that isnt used by the ssd? Of course. That isn't the argument here. Likely they had to increment by 2gb so couldn't just give it 5. The ssd might only need an extra 300Mb of ram to stay performant, but that is still 300mb more than the 4gb would have given it.
Is this really harder to believe than Apple mysteriously giving one sku out of 10+ more memory just in case a dev felt like developing for it? How many apps are even made exclusively for ipad pro let alone just one device amongst them. If there is a side benefit that apps stay resident longer in ram, thats good, but it still isnt going to be actively exploited by applications until the rest of the ipads get ram boosts too.