Search Engine Journal has a useful post analyzing the latest Google PageRank Update in which they speculate as to why many sites have decreased in their visible PR whilst increasing their backlinks:
‘The most current PageRank update will undoubtedly cause a larger stir than usual in that many sites have shown drops in their visible PageRank while at the same time showing significant increases in their backlinks. This fact reveals that one of three things has occurred in this latest update:
1. Google has raised the bar on PageRank, making it more difficult to attain a high level, or
2. The way they are displaying their backlinks has changed, or
3. The way they calculate the value of an incoming link has changed.’
