Aaron Wall wrote a blog post titled Why Do Keyword Tool Search Estimates Vary So much?. In his post, he suggests that the tools should serve as a way of identifying trends in various keywords, rather than being relied upon for hard numbers. According to him, the keyword suggestion tool should be used for more qualitative purposes.
He advices to look at keyword depth, related words, and reasonable modifiers. "Depth and related words matter far more than just the sheer volume for a generic term because the longer queries are more targeted and thus easier to monetize, and longer search phrases are typically easier to rank for than shorter keywords."