Sheara Goldenthal pinged me with a question yesterday about why the Google Keyword Tool shows different search volume data for the same keyword phrase, when she is logged in versus not logged into her AdWords account.
It is a great question, a question I thought I knew the solid answer for. I am sure I read it somewhere, so I dug a bit deeper. It seems like this is a popular question asked in several forums, including HighRankings Forums, Warrior Forums and several Google AdWords Help forum.
There is a help document that answers this question:
Why do traffic estimates for my ad group differ from those given by the standalone tool?
The results yielded by the Traffic Estimator in your ad group and the standalone Traffic Estimator may differ for two reasons:
(1) Performance Data: The past performance of an ad group will influence the Quality Score of all the keywords in that ad group. Your ad group's Traffic Estimator considers the ad group's performance details when making estimates about your keywords. The standalone tool, on the other hand, does not. For the most accurate results, use the Traffic Estimator in your ad group.
(2) Double-Serving: In accordance with our double-serving policy, we will only show one of your ads on a search for a particular keyword, even if that keyword appears multiple times in your account. The Traffic Estimator in your ad group accounts for double serving among your keywords when making estimates, whereas the standalone tool does not. Therefore, the standalone tool may yield higher estimates when it evaluates a keyword that appears multiple times within your account.