Conflict between System.setProperties() and System.getProperties() API specs

The API spec for class System has a contradiction between System.setGetProperties() and
System.setProperties(). getProperties() asserts that some properties will always
be available, while setProperties() asserts that it can overwrite ALL of the system
properties(). Which is it?

The spec for System.getProperties() reads in part:

       This set of system properties always includes values for the following keys:

             Key
                        Description of associated value
        java.version
                        Java version number
        java.vendor
                        Java-vendor-specific string
        java.vendor.url
                        Java vendor URL
        java.home
                        Java installation directory
        java.class.version
                        Java class format version number
        java.class.path
                        Java classpath
        os.name
                        Operating system name
        os.arch
                        Operating system architecture
        os.version
                        Operating system version
        file.separator
                        File separator ("/" on Unix)
        path.separator
                        Path separator (":" on Unix)
        line.separator
                        Line separator ("\\n" on Unix)
        user.name
                        User account name
        user.home
                        User home directory
        user.dir
                        User's current working directory


The spec for System.setProperties() reads in part:
The argument becomes the current set of system properties for use by the getProperty method. If the argument is null, then the current set of system properties is forgotten.