Forum Moderators: open
Yes, previous linking sites may have droped in pr, or you may have lost one high pr link and gained several lower ones!
Probably many previously counted links from directories have either now been ignored or dropped in pr.
As Thomas says, the google link tool is a mere snapshot, not a complete picture. Try alltheweb or others for a better picture.
High PR is a bit like having many clothes in your wardrobe, but each are relevant for different situations.