They also mount wide-- in the Thickslicks a 23 is really more like a 25, and that's before mounting. They "underspec," basically.

Edit: OP, what is the inner width of your rims?

If you have close tolerances on 25s, I would recommend you try your LBS and see if the tires will actually fit. The non-Elite Thickslicks are some of the cheaper road tires, most bike shops will have them, and a good bike shop will mount one up and not make you buy it if it doesn't clear the frame.