Google sitemaps

Crawl statistics page

[apprentice]

I was looking at the crawl statistics within my sitemap account the other day:

Your page with the highest PageRank
Sep http://www.example.com/
Aug http://www.example.com/
Jul http://www.example.com/examplepage1/

These seem to update occasionally as sometimes it reports for the last month "no data available at this time". What I thought was a bit strange is that the page with the highest pagerank in July was http://www.example.com/ - and indeed it was at the time as far as I can remember. But with the latest update it now says it is http://www.example.com/examplepage1/

Does anyone know how these are calculated? Are they just estimates, are they generated dynamically or what?

Regards.