I think one of peculiarities in Brooks saddles is that they are basically hand made. This will almost certainly insure each one can be unique (if only on the smallest of scales) from every other Brooks in the same model. Personally, I do not see any difference in quality between the Professionals I have had or still have, from a late 60s model, to '70s, '90s, to the Alpe D'Huez which recently arrived.

Now different models of Brooks have less hand crafting than others, which allows a small steel rivet B17 standard to be less expensive than a hand hammered, large copper rivet special edition racing green B17 with copper rails. The other variable are cow hides. Hides are graded and a more costly hide used on a Professionals different from a lower grade hide used on a B17 Standard.

Let's not forget that Brooks England is a division of Selle Italia, and the bottom line is they need to be a profitable business. Selling lots of affordable B17s makes it possible to offer other saddles of a significantly better quality. A corollary might be how Ford sells significantly more entry level Focus models than they do top of the line Lincolns.