On the surface, you may not see a difference. But if you analyze comb utilization, the area that a clusters can keep warm, and other aspects of not wasting space in the brood chamber, then there is a huge difference in amounts of brood raised, which in turn effects many other things.
Bees in the brood chamber only need the minimum cell depth required to produce brood. Excess bee space is wasted resources and cuts into production, etc.
If bees can only cluster a certain size, or a have certain volume in regards to what the beekeeper dictates, then why lower the comb cell count?
I can explain why 9 frames instead of 10 can be a detriment. But there is no reason or benefit that can be shown as to why 9 is better or even non-impacting. There is in fact a negative impact when the comb is spread out beyond what the minimum dictates.
The same forces at play that dictate 10 frames in the brood chamber best, are also seen in reasoning why 9 would be better than 10 in the honey supers.