Right. B17's have a reputation to be made of thinner leather than Professionals, so tend to be/get softer; but Brooks literature makes no mention of this that I've seen. From my own (pretty limited) experience working with leather, the way the leather was treated at the factory could account for a difference in hardness as well. But again Brooks literature makes no reference to differences in the leather between the two. There is a pre-softened leather option, which is, I presume, the result of a different process. That is, I doubt the saddle has been softened, but rather it was never made as hard.

What is certain is that use changes leather, so different use will result in different leather. Proofide, sweat, dew, sun, rain, and riding will all make a difference one way or another, and (unless you witnessed all these things yourself) in the end all you know is how the saddle turned out.