A: You had asked for the company to offer you a salary of $16,000, yet they gave you $17,889, nearly $2,000 more. Your company clearly has a minimum rate it is willing to offer for this job, a rate higher than your initial salary expectations. I wouldn't characterize their offer as unfair, since you disclosed a salary figure first.
Now, your expectations have changed based on new information. But you have already accepted the job at this rate. So what can you do to increase your salary, given what you know today?
You may be able to get an increase based on the information you found in the Salary Wizard. Ask the human resources department when your job was last evaluated (that is, when the company's compensation team last determined the job's market value) and whether there is a process for evaluating the job in the future. This is a bit of a long shot, though, since you have already agreed to the salary you are earning now.
Your best bet may be to ask the company what other career opportunities they have that would enable you to make more. And if they show you a list of openings, whatever you do, don't be the first to say a number.