I have a site that ranks 3rd for the term "e decorating". According to google's keyword tool that gets 12,000 monthly impressions.
However, according to google's webmaster's tools, I'm only getting 35 impression for that term. Why is this?
The most likely reason is that the site only shows up third for you. There are many factors that determine search placement, including your location, previous searches etc.
I also believe that the 12,000 number includes searches with other terms, eg "e decorating website", "e decorating in [location]" and so on. So there may be 12,000 searches globally, but you only show up for a small number of them.