Question about google page results

This may be covered somewhere, but I haven't been able to find any info. I was wondering why when I type in a search query into google and I go to the back page the number of results drop.

For example, I type in "google page results" and I get 66,500 pages. I go to page 11 and the page results are the same. Then I go to page 12 (the last page) and the results drop to 113 and It gives me this :

In order to show you the most relevant results, we have omitted some entries very similar to the 113 already displayed.
If you like, you can repeat the search with the omitted results included.

I click on the link and the results return. Are 66,400 pages that similar?

it will usually be multiple indexed pages of one site. so if my site has 100 pages indexed by G, there will be 100 almost the same results. this obviously depends on the content of the site (if the site has varied content, there will be less duplicates)

a good example of this is:
i post tons of ads on gumtree. they are all almost the same. the ones that haven't been deleted have been indexed by G. since the root domain is the same on all of them (gumtree.xxx), and the content is the same, G see's this as being pretty much the same thing.