About three months ago, we reported that the Google AdWords API Traffic Estimator figures were off. Since then, nothing much has changed, but Google announced that they have made changes that should help stabilize those figures and make them more accurate.
A Google Groups thread has a post from AdWords API Advisor saying:
I wanted to follow up on an issue that was brought up a few months ago. The engineering team has recently made a change to the Traffic Estimator service that could address the issues some folks were reporting regarding "spiky" results that varied too much over the course of several requests.
There has been no confirmation yet from AdWords API users that the numbers are fixed and looking to be accurate.
Forum discussion at Google Groups.