Why are unoptimized pages at the top of google?

There's something that Google does that confuses me! I have some pages that use competitive keywords and have no backlinks. The thing is...some of them are at the top of google.co.uk and google.com. Any idea why this could be? The only thing I can think of is that the pages have a very high click through/conversion rate. I have a theory that google might use this when deciding where to place a page on it's index.

Now, there's some pages on the site that are well optomised, have less competative keywords and good, relevent backlinks. Yet, some of them barely scrape into the top 10. Typical!

Some insight from thos in the know would be great!