[X Newbies] decrypting keychains

[TheMacintoshLady]

cricket tapped out this message on 8/26/03 3:03 PM 

>The reason you get the warning dialog when you upgrade your system is 
>that keychain keeps track of which applications have access to which 
>keychain items -- and when you upgrade, the application has changed and 
>it is no longer 'trusted'. For example, would you want it to be 
>possible to write a program, name it 'Mail', and have it snag all your 
>Mail passwords?

I understand this, but why does it do it when Keychain was not in use in 
the first place? Am I musunderstanding something in the above explanation?