What is your concern?
I would expect most actual users of the Pro model, now that 13 inch is available at the lower tier, would be working with photos and video. Even shooting ProRes off a pro iPhone is going to eat into 256 pretty fast.
Seems like that model exists mainly so they can charge $1500 for the one people are actually likely to get, and still say "starts at $1299".
Then again, it's Apple, and they can get away with it, so they do. My main point here is that the 256GB model is bad value compared to the equivalent Air model, because if you have any work where the extra beef is going to matter, it's going to eat right through that amount of storage pretty quick.
That type of buyer is a very significant % of sales for iPad pros. Despite the marketing, there are really not that many people (as a % of sales) that will be pushing these iPad's anywhere even remotely close to their computational/storage/spec limits.
Unless you are a solo editor.
My guess is they want you to use their cloud storage and pay monthly for it.
I'm not storing my Docker containers and `node_modules` in the cloud.
Pro isn't just images and videos.
I said this in a sibling comment already, but I think it's just price anchoring so that people find the $1500 they're actually going to pay a bit easier to swallow.
That said, Apple's policy toward SKUs is pretty consistent: you pay more money and you get more machine, and vice versa. The MacBooks are the only product which has separately configurable memory / storage / chip, and even there some combinations aren't manufactured.