7-speed freehubs are designed with 126mm spacing. 8/9/10 speed freehubs (which, from Shimano, are interchangable; an 8/9/10 speed cassette fits on the same sized freehub) are designed with 130mm spacing.
Specifically, steel or titanium frames are flexible enough to spread the frame that extra 4mm to run a 130mm rear hub in a frame designed for 126mm.
But alternately, you could take a 130mm-spaced hub, remove a washer or two on the non-drive side of the axle, and get 126mm spacing with a slightly re-dished wheel. Not an ideal solution (results in a weaker wheel) but it works okay, actually. Provided you're quite good with wheelbuilding. If you do this with a Shimano hub, it's not that much different than a Campy hub with unmodified spacing (Campy hubs have more dish than Shimano, because of the flange placement).