When you are in a namespace in the UI/CLI, artifacts from the system namespace are available to you as well. However, the metadata search API is only restricted to entities within your current namespace.
A typical example - You are in the default namespace, list artifacts shows you cdap-etl-batch and other system scope artifacts. You can even retrieve the metadata of the artifact using the get artifact metadata command. However, search results can't match the cdap-etl-batch artifact.