in piwik one has access to two different statistics for visits, the “visits overview” and the “overview”. as expected the values for visits are the same in both statistics (see screenshots: “70 visits” ). interestingly, the values for unique visitors differ (“27” vs. “44+4+3” = 51 visits).
In the “Visits overview”, we have 503 visits and 448 unique visitors, respectively. In the “Overview”, we have 503 visits and 490 unique visitors, respectively (see attachments).
I think it makes sense, because a same visitor can be once a direct entry and once a search engine for example, in the same day. So it will be counted as 2 when summing the unique visitors in the referrer report, but the groups are not exclusive.