SLR viewfinders are optimized for to show brighter images when using zoom lenses. High(er)-end SLRs are usually optimized for faster zooms (~f/2.8 ) while low(er)-end SLRs for slower zooms (~f/4). A side-effect is that the DOF of the image you see in your viewfinder is limited to the f-stop for which the viewfinder is optimized.
Using your DOF preview button, you can stop down your lens' aperture until the viewfinder begins dimming; the last stop for which no change occurred is the f-stop your viewfinder is optimized for — probably f/2.8 on your 5D. Even though the image in the viewfinder has the DOF and brightness of a stopped-down lens when shooting wide open, your lens' aperture is actually wide open at this point, so if your lens experiences focus-shift, you're viewing the focus point at maximum aperture, not the reduced viewfinder aperture.