A Google Groups thread reports that the numbers reported from the traffic estimator tool within Google's AdWords API interface is reporting different numbers based on the IP calling the data.
The AdWords advertiser said that depending on which IP is requesting the traffic data, Google will show a different traffic estimation for the same keyword.
For example, if the AdWords advertiser wants to know how much traffic to expect for the keyword [car], if he/she uses the API to request the data, the data might be wrong. If the API is requesting the data from IP address xx.xx.xx.xxx, it might return one number. If the API is requesting the same data from IP address yy.yy.yy.yyy, it might return a different number.
Jeff Posnick from the AdWords API Team said he will investigate the issue:
Thanks for letting us know; our engineering team is currently investigating to see if there is anything that might have changed recently that could lead to varying results from the Traffic Estimator API calls. I'll update this thread with what I hear back.
I'll update you when I hear more information.
Forum discussion at Google Groups.