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

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.