WN, I agree it's VERY confusing...
I'm definitely no expert but I think as with every brand new H bag, it will be a little "stiff" initially, with a bit more use, it will soften or slouch. As to how much it will slouch - that's where the construction comes in.
About a year ago, I asked a Hermes SA what type of leather would make a sellier Kelly ( as in my intepretation of
Sellier = outside stitch + soft leather ) and she said Gulliver. But she added that Hermes had discontinued Gulliver and sellier Kelly are not common.
With the come back of Swift and the Kelly Flat (that foldable Kelly), maybe we'll see more of of such Kellys..