I know, but different output is a different sound, even if it's just a little bit.
You also have to remember, the wire on them is different, not much different, but different enough. The 57/08 is most like the wire used in 1957, the 59/09 is most like what was used in 1959 and 53/10 is most like the wire in 1953. While not a lot of different, different enough to be classified that way.
I think the main reason is that the wire for the 57/08 is also used in the Narrowfield (when they were still made) and the 408's and a few other I think. It seems they just use more of that wire for their own pickups and it might just be a simple supply and demand thing. They don't have the supply on hand to make just pickups to sell.