Webmaster Tools...do they lie?

Hi

I've got various websites and within Google Webmaster Tools there is a section called "your site on the web" and then "search queries" within this section it lists you average position for popular keyword associated to the website selected.

Now for all my sites these averages are fairly accurate and give a nice indication of how my work and SEO progress is coming along.

However my most important micro-website:

mobile-shelving.com

For the keyword "mobile shelving" is meant to have an average position of 39 yet it doesn't appear anywhere for this key word! Other keywords such as "rotary shelving" appears within a few spaces of 16 as stated by Google to be the average!

So why for "mobile shelving" is my website not ranking where they say it should be?

I am not one to defend the reliability of Google's webmaster data, but the idea is that the average ranking recorded is "aggregate", i.e. based on how your site has appeared in the SERPs in various Google datacenters and also in the various regional Googles (such as Google.com, Google.fr, Google.co.uk, etc.). Rankings vary considerably among these. Nor are they stable.