I don't think that there is any particular rhyme or reason with page ranking. It's a hit and miss affair. Think Google uses a special algorithm and changes the recipe all of the time. The algorithm is used to count the number and quality of links to a page to determine a rough estimate of how important the website is relative to the Web.
By the way. It used to be that one could find Websites with 10/10 rankings. Like Facebook and Apple. However looks as though every one including Google is down to 9/10. Wonder which ones are still 10/10 or have been upgraded to 10/10? Can't believe it, Microsoft is 8/10. NASA is 8/10 !!!! :shock: