Established Site not Ranking for one particular keyword

My site, statisticshowto.com, has decent traffic, however, it doesn't seem to be ranking for the one keyword I would have thought it would: statistics how to.
According to Google keyword tools, 20.400.000 global searches are performed for "statistics how to". And yet, Google Webmaster doesn't show this keyword as ranking for me at all.
When I perform a Google search for "Statistics how to", my site comes up top.
I would think that after almost 3 years and pretty decent traffic (I've grown to 150,000-200,000 page views in the last few months), that I would be ranking for this keyword.

Any idea why I might not be ranking for that particular keyword?

Thanks!