Actually, there are quite a few exceptions to that rule. Duncan F-spaced are 52.6mm. My Patriot's hardtail is a whopping 56mm, and the new Hipshot hardtail that I just bought is 53.5mm. And that was the narrowest aftermarket hardtail I could find.
It seems to me that the "standard" spacing of Duncans, (and many pickups), is more rare than F-spaced is.