System.setPassword()

Hi All,

i have seen this from saleforce help
-> The last three passwords are remembered and cannot be reused when you are changing your password.
Please check this link, https://help.salesforce.com/HTViewHelpDoc?id=security_overview_passwords.htm&language=en_US

But the method System.setPassword() is not follows the above limit. I have used this method to set the password for portal users and this function used to reset the alredy used password
For example:
1 time password is demo001
2 time demo002
3 time i have used again demo001 then not showing any error its restting the password with demo001

Please advise me why is working, tell me any setups to aviod this

Thanks