Last Visit Graph:the data range is year, the number of visitors is not right


#1

In the date range,I chose year,and the last visits graph shows the total count of each years.
My website be visited since May,2011.
In the database,the piwik_log_visitor is 146,but in the visits graph it showed 138 in 2011.
I used piwik1.2.1, There is something wrong with my piwik??

It would be highly appreciated if you can tell me why.

Jocelyn