It would be far super-weirder-super-odder-super-rarer if these twins had different moms. — Why do we regard a lock that can be opened by any key as a weak lock

It would be far super-weirder-super-odder-super-rarer if these twins had different moms. — Why do we regard a lock that can be opened by any key as a weak lock, but call a key that can open any lock a "master key"?