The unique visitor/returing visitor counts are a little off.
The unique visitors and returning visitors is based entirely on cookies. If a person's browser has cookies disabled they will always be counted as a unique visitor. If they delete their cookie and visit your site again they will be counted as a unique visitor and not a returning visitor.
It can never be an exact science with determining the exact number of unique visitors but it does give a very good idea, and you can easily spot trends over time from this information.
Answers to the majority of questions we receive can be found here in the knowledge base.