lhalcong: I think you are getting confused between exposure and developed densities. It seems like you are using The Negative by Adams. Re-read the section on characteristic curves, exposure, density etc.
Note that a step tablet/wedge is an exposure device for testing. It is not intended to tell you what the resulting densities of the developed film will be. Exposing film through a step tablet allows you to give the film known increments of exposure. That's all it is doing.
Here are some easy to follow publications which may help to start you off: