Wheel sizing is typically based on the tire size intended to go on that wheel - it does not usually reflect the actual dimensions of the rim itself. This is really the opposite of how car wheels/tires are sized, and it confused the heck out of me when I first got into it - especially when compounded by trying to comprehend the many different rim diameters that are considered 26". Rim widths do vary, even for a given designated tire size. For example, take a look at Velocity's rim catalog. They're nice enough to publish the width and height of each of their rims.
You can often get away with deviating from the indicated tire size by a bit - a narrower rim will make the tire taller and a wider one will make it wider. Also, Sheldon Brown wrote an article on matching tire widths to rim widths, but I think that his advice was overly conservative by today's standards. MTB guys are a good example, they often run very wide tires on quite narrow rims.