sharepoint write security not working

Hi all,

I am using the following powershell code to set item level permission to sharepoint document library..

$web = Get-SPWeb http://testserver:1007
$list = $web.Lists[“Documents”]
$list.ReadSecurity = 1
$list.WriteSecurity = 2
$list.Update()
$web.Dispose()

But the users able edit the documents created by other users.. Anyone know what is the reason for this issue..