The main cause of this problem would be using other authentication methods rather than Mac’s default local login settings. Sometimes, it can lead to some problems when the Keychain password does not match with login password. And, when the first time you start Keychain app, it’ll have the same password as your Mac’s login password. So when you open a website that ask your login password in your Mac, Keychain app can provide the password if it’s on its password list. This utility can store all the passwords for your applications, websites you visit and even the sensitive information including credit card numbers or personal identification numbers for bank accounts. Most of the Mac users would be either used or aware of Keychain – Apple’s password management utility on Macs.
