By default you see the logins for the previous calendar month. If you view this page in April, you will see the results for March.
To alter the results time period you can add a 'start' and an 'end' option to the URL, e.g.
'admin/users/statistics.php?type=logins&start=2015-01-01&end=2015-04-30'
This will return the results for the 4-month period of 1 January to 30 April 2015.