Incongruent statistics

Piwik vs Google Analytics: which is counting the beans correctly?

Website/visitor stats are just simply fascinating, but I do not know enough about them to analyze them correctly.

Can you help me?

The screenshots below are from Piwik and Google Analytics, both from the period of August 19 – September 7, 2011. The incongruity baffles me.

Piwik stats : Aug 19 - Sep 7, 2011

Piwik stats : Aug 19 – Sep 7, 2011

Google Analytics stats : Aug 19 - Sep 7, 2011

Google Analytics stats : Aug 19 – Sep 7, 2011