The metrics that I am tracking for this site come from multiple, publicly available metrics. The two main tracking tools used on this site are Alexa.com and Quantcast.com.
These metrics are collected using various technices. This also means that there will be diffrences in the reporting mechanisms. The strenght and accuracy of US when compared International tracking plays a big roll in the differences.
This can best be shown in the tracking of NotreFamille.com. Alexa.com has NotreFamille.com as a very high traffic site, while Quantcast has it as a very low traffic site.