To elaborate a tad:
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.
The extra space between frames is essentially wasted and allows pests such as SHB and wax moths easier access to the hive and creates a situation that makes it harder for the bees to police the interior of their own home.
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.
How many cells are there on a frame? Every frame removed from the brood chamber reduces the population the hive can produce which weakens the hive, the extra space is also an impediment.
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?
Lower cell count (less frames) shorts the bees of needed winter stores amoung other things. This honey and pollen stores equates to a greater propensity for hive loss. Brood chamber cells are of a given length regardless of the number of frames because the pupae only gets so large and any cell siize larger than that just won't be built. The shorter combs plus the greater space between frames dictates less stores. Less frames also enlarges the open air space within the hive. A healthy hive needs less open air space to cluster space iin order to survive a harsh winter so 9 frames delievers a double whammy.
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.
9 frames in a harvestable honey super can only be a possible benefit to the beekeepers. Any imagined advantages of 9 frames is just that: Imagined.
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.
A debateable benefit, as my experience and observations have been that the beekeeper can harvest more honey from a 10 frame super than a 9 frame super and he doesn't have all the associated burr comb and pest problems that a 9 frame configuration induces. Too often in 9 frame configurations the combs on either side are drawn out to such a depth that the frame between is not utilized at all....that is not an advantage.