Is that not because the time gap between the disconnection of the users and the display of the user as online ?
Admin Panel - Advanced Settings - Time period in minutes within which a member is considered to be online
Setting a low value for this option will show really online users
Setting a high value for this option will show really online users and already disconnected users
Admin Panel - Advanced Settings - Time period in minutes within which a member is considered to be online
Setting a low value for this option will show really online users
Setting a high value for this option will show really online users and already disconnected users