Just IMO because I have no technical knowledge .... but regardless of how hard or soft the steel is there may be a difference in abrasion resistance. One of the reasons that 304 stainless is used in cement plant klinker cooler sifter plates is because of the abrasion resistance. One of the reasons some SS blades have a reputation for requiring more strokes.
I don't know anything about forging but I'm betting that the smiths at W&B used the eyeball method rather than pyrometers on those early blades. It is proven by US Army tests of faulty Springfield '03 receivers that using the eyeball to judge forging temp and duration by color has variable results. So maybe all of the old Sheffields aren't as uniform in hardness as the twentieth century razors ?
Finally there is the amount of steel that needs to be removed to achieve a bevel. Way more on a Sheffield wedge than on a full hollow. It is not by accident that the full hollow came to dominate the market in the late 1800s. I think the fact that barbers and the general public embraced them at the time points to the much easier honing and maintenance of the full hollow. Until the custom guys came along if you wanted a true wedge it was vintage or nothing. The full hollow drove them into obsolescence.