Password Synchronization
If the login keychain is protected by the login password, then the keychain's password will be changed whenever the login password is changed from within Mac OS X. However, on a shared Mac/non-Mac network, it is possible for the login keychain's password to lose synchronization if the user's login password is changed from a non-Mac system. Some network administrators react to this by deleting the keychain file on logout, so that a new one will be created next time the user logs in. This means keychain passwords will not be remembered from one session to the next, even if the login password has not been changed. If this happens, the user can restore the keychain file in ~/Library/Keychains/ from a backup, but doing so will lock the keychain which will then need to be unlocked at next use.
Read more about this topic: Keychain (Mac OS)