Because your bag is so dry, it could have been stored in an attic, or even an outside storage building (grandpas barn or shed?), warehouse, etc. That's the part we aren't usually aware of unless a seller happens to mention that the bag had been stored. The seller of the dry rot bag I got mentioned that she was going through her mothers' things and the bag had been stored. But that was when I contacted her regarding the condition.
In hindsight, the Nubuc I saw recently (THANKS again ladies for the "devil" warning!) had been stored. I only knew that because I messaged for additional info regarding the condition and more photos. The seller mentioned it had been in storage and was dry - that alone should have been my warning!!
Actually the same goes for mold. Those bags could have been stored in a box or container sitting on a concrete garage or basement floor which drew in moisture.
Since many sellers on eBay, etc are not the original owners, or even collectors in some cases, they likely do not know (or care) how or where the bag spent it's previous life.
Wondering if there is such a thing as early stage dry rot? Could that be the reason some bags are dying of thirst and soak up the condition rather quickly, needing many coats during the rehab process?
Rehabbers like a challenge, but.....
BTW - was your seller any help?