When adding data to a piwik installation via the server log analytics script, visits and unique visitors (almost) don't differ. No matter which date range is selected they keep almost the same. Looking on out user structure, there should be a lot of recurring visits.
29143 vs. 29142 (day)
87041 vs. 87038 (week)
Server logs are in standard apache format and rotated and processed every early morning.
In the config.ini.php you can customize this setting
[Tracker] ; The window to look back for a previous visit by this current visitor. Defaults to visit_standard_length. ; If you are looking for higher accuracy of "returning visitors" metrics, you may set this value to 86400 or more. ; This is especially useful when you use the Tracking API where tracking Returning Visitors often depends on this setting. ; The value window_look_back_for_visitor is used only if it is set to greater than visit_standard_length window_look_back_for_visitor = 86400
Thanks a lot! Problem solved.