WordTracker shows the approximate number of DAILY searches. Overture, or any tool using Overture's database, shows the approximate number of searches done in a month. In addition, the numbers will vary as each tool queries different sources to gather this data. Overture's numbers have been said to be inflated due to overuse by webmasters, just FYI.
Here's the link to the Overture tool, for anyone interested:
http://inventory.overture.com/d/searchi ... uggestion/
... and for the WordTracker free trial, the SSWT link is:
http://www.selfstartersweeklytips.com/wordtracker.htm