depending on the browser configuration of the reader, we are able (or *not* able) to see, from which website he or she came from.Some users use some sort of "private surfing" on which such data is not transmitted to the hosting servers.
So, the numbers do not reflect a fully correct value, however, they are representative. The percentage of each source should be more or less correct.
I have just changed the text a bit to "For technical reasons, not all visits are depicted".
If you are interested in more analytical data, I would recommend Google Analytics, which can be used with Wizzley - and it's all for free :)