The javadoc of jdk.vm.ci.services.Services.getSavedProperties is currently:

    /**
     * Gets an unmodifiable copy of the system properties parsed by {@code arguments.cpp}
     * plus {@code java.specification.version}, {@code os.name} and {@code os.arch}.
     * The latter two are forced to be the real OS and architecture. That is, values
     * for these two properties set on the command line are ignored.
     */

The details about how the copy is initialized are specific to the HotSpot VM. On SVM, the semantics are slightly different. This should be clarified in the doc.