Google provides monthly traffic estimates by KW in both its original Adwords Keyword tool (ADKWT) and its newer Search-Based Keyword Tool (SBKWT).  In some cases the numbers reported are similar, but in most cases the monthly traffic numbers appear to very different, with the SBWKT numbers being only a fraction of the ADKWT.  Here are some examples taken from a "business intelligence" search with each tool. 

KW, ADKWT, SBKWT

• business intelligence bi, 4400, 33
• business intelligence books, 720, 350
• business intelligence cube, 320, 125
• business intelligence kpi, 210, 52
• business intelligence metrics, 170, 52
• business intelligence performance, 880, 52
• business intelligence planning, 320, 66
• reporting services analysis services, 590, 100

Changing the Adwords KW tool to exact or phrase match can drastically reduce the estimated average monthly search counts reported, but this does not seem to help align the numbers.  It appears to swing the Adwords numbers below the search-based KW tool numbers.

I tested this with the search-based KW tool running across several different sites in the BI industry, and the search-based KW monthly searches reported was invariant. There are also great differences in the competition level reported for each KW, but the differences in the search counts are more puzzling.

Perhaps there is a better explanation online, but I haven't found it yet.  It would be helpful if Google could provide clarification on what the numbers actually mean.  The odd thing is that, in practice, we use the Adwords KW Tool numbers, but factor them down to a fraction to account for market segmentation and relevance.

For example, "call blasting" is a core KW for one of my customers in the voice broadcasting and automated phone message business.  However, a large fraction of "call blasting" traffic relates to people who take large amplifiers and speakers out to Pacific northwest forests and play "Sasquatch calls" to lure Bigfoot.  So, -Sasquatch -Bigfoot, and we'll hope to learn more later about why the traffic numbers differ.