Each tool is probabaly accessing a different Google data center. Your Google toolbar maybe accessing a Data center on the east coast, and the keyword tracker maybe be on a server located ont he west coast accessing a closer Google data center. Just wait until they are all updated to the same data. At least that's what I'd think it is.
First off: http://www.vokaboly.de and http://vokaboly.de are considered 2 different sites. That's why they have the different PR. and yes the only time PR fluxuates is when Google is updating, but that isn't the case here. without the 'www' in front you will get a different PR for that site.
My guess is the site has had 2 links listed since it was PR0. The keyword tracker only updates PageRank (and pages in URL for that matter) when the back link number changes.