The estimated visits in the keyword research workspace appears to be somewhat out of date and inaccurate.
Looking at the numbers it would appear you are using the old 42% CTR for a site ranked #1 for that key phrase, however some recent research shows that the actual click through is only half that.
After all since the research which suggests 42% ctr for a #1 ranked site there has been many many changes to the SERPS.
Details of the new research can be found here:
http://www.seomoz.org/blog/mission-impo ... ough-ratesI look forward to your comments and hopefully a change to your calculation when generating these figures

Danny