Google mentions that the values include the number of searches from the search network (sites like Ask.com, AOL, Netscape) and they're approximate. "These approximate numbers are intended to provide better insight into keywords' monthly and average search volumes than previously provided by the tool. (...) The Keyword Tool also provides several other keyword-related metrics that can help you select highly relevant keywords to improve the overall performance of your campaigns. You can easily view data on advertiser competition, search volume trends, estimated average CPCs, and estimated ad positions for keywords," mentions Inside AdWords blog.
