Its the real world
I am always surprised at peoples reactions to this type of research. Unfortunately organisations have "pay bands" and brackets, so if your good at your job you may still only earn the same as a collegue who is shockingly bad. Unfortunately thats life, not just in IT.
Obviously were your business is located will also have an effect on your wage, aswell as other factors, such as your companies size (ability to pay more), size of customers, size of contracts, your responsibilities etc etc etc.
My own company pays purely on performance and ability. If we have a great help desk worker who works hard, fixes lots of issues he will always get paid more and be up for promotions compared to those who dont perform as well or those we feel dont have as much knowledge or ability to learn more. The key is to ensure you can monitor performance well enough...
Finally noticed some responses with regards to women. I think that women often work a lot harder than men (in general not always obviously) and they are more often than not, more disciplined in their working. In many cases I am sure women work harder than men just to get the same recognition, unfortunately too many in IT see this industry as a very male orientated one....its simply wrong...
Biting the hand that feeds IT
