Why Google-PR gives Drupal 8, Joomla 9 and Phpnuke 5?

I just dont understand how Google-PageRank calculated. It is nor pure popularity either pure quality.


link:drupal.org 323,000 PR 8
link:joomla.org 635,000 PR 9
link:phpnuke.org 452,000 PR 5
link:postnuke.com 1,110 PR 8

Phpnuke has more backlinks than Drupal but very low PR
Drupal is more professional than Joomla but lower PR
Postnuke ended at 2008 but still have PR 8