I know that this is an old thread, but I must say that it's EXACTLY what I was looking for!!!
I too have been puzzled by the Competitiveness scores produced by different tools (Wordtracker, MOZ etc). Many of the keywords I've researched in my particular field (refurbished laptops and computers) show Low Competition in these tools, but when I perform a regular search on Google, the first two pages are often populated by big name brands (Amazon, Bestbuy, NewEgg,Walmart, Sears, Overstock etc.)... which I assume would be difficult to rank above.
The interesting thing here is that these big brand pages don't often score high on backlinks, In-Anchor-and-Title or other Competitiveness indicators, but yet, they rank high on the SERPs.
I'm definitely a newbie at this, but this article puts the missing pieces together for me.
THANKS!