I'm no 3D math guru, but AFAIK, with a zbrush (or mudbox, or 3dcoat) workflow, you use normal maps for "trick" geometry perturbation (as in percieved but not real), and you use displacement maps to physically perturb the geometry. No bump maps.
Or in other words, you could also work with two greyscale maps, one for the bump, and another for the displacement, but they would be different maps. So in this case, normal maps are probably used so you don't get confused as to which is which.
Just my 2 cents.

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