Due to immense amounts of data that Ahrefs bot has to crawl/re-crawl constantly, its algorithm includes certain priority rules. 

Because of that, there is a specific list of pages that we have in our index.

If you open any site in Site Explorer, you can see Crawled pages count on the right side:

This is the exact number of pages currently in our index for the mode you chose (domain with subdomains, domains without subdomains, exact URL or prefix).

Now, if you want to see what those pages are, you can go to Best by links report in Site Explorer: