I found an error in piwik 1.12 with date ranges.
My site has two referer witch has sent me more than 2.000 Visitors yesterday.
If i look at the report for “yesterday” everything looks fine. Now i took ranges like “last14” (via API) or “2013-05-21,2013-06-10” and the number of visitors is lower than 2.000 for both referers. So i took “2013-06-09,2013-06-10” and the number of visitors seemed to be legit. I found out, that if i take a beginning date older than 7 days (or higher than “last7”), the visitors from yesterday are not shown.
The data-graph is showing the right visitor counts for each day.