Looks like it's going on scales of 10 instead of 1. I have a PR1 blog that shows a ranking of 10, and I know Twitter is a PR10 site so it's showing PR100. I'm not sure why Foursquare would be a zero... did you spell it correctly?
I have no interest in PR. It only matters when PR is greater than 7 or 8. Otherwise, you can have a PR 0 site ranking just as high as a PR 5 site.