Even now a day we can not trust on Google also, lol. Google has changed it's way for distributing PR.
the PR is just a representation of how many links you get to your website. If you do lots of work you will get a good PR. I just use these tools for a bit of fun. Sometimes they are right, sometimes they are wrong but I don't get disappointed because I only get what I deserve. If I don't get a good PR it simply makes me work harder for the next update.
These tools are not good. Last week I bought my domains and I tried to see what pagerank will it have. It had no backlinks and a tool predicted that it will have pagerank 5 ). So don't waste your time with these.