Entities from Google Vision API have no infromation in Knowledge Graph API

Question

I use the Google Vision API to detect themes in images, and I then use the Knowledge Graph API to retrieve information about those themes (such as a description, a thumbnail, and a Wikpedia link, for instance). However, I sometimes find that entities returned by the Vision API have no information whatsoever in the Knowledge Graph.

Here is an example.

Using Google Vision API Demo, analyze this image.

In the returned JSON, you can see "Atmospheric Phenomenon" as a detected label, with its Knowledge Graph identifier.

{
  "labelAnnotations": [
    {
      "description": "Atmospheric phenomenon",
      "mid": "/m/07pw27b",
      "score": 0.874821,
      "topicality": 0.874821
    },
    // ...
  ],
  // ...
}

You can verify that the entity exists, by searching it directly on Google via the following URL.

http://g.co/kg/m/07pw27b

However, when using the Knowledge Graph API to fetch information about that entity, the response is empty.

{
  "@context": {
    "EntitySearchResult": "goog:EntitySearchResult",
    "resultScore": "goog:resultScore",
    "@vocab": "http://schema.org/",
    "kg": "http://g.co/kg",
    "goog": "http://schema.googleapis.com/",
    "detailedDescription": "goog:detailedDescription"
  },
  "@type": "ItemList",
  "itemListElement": []
}

---

How come that certain entities returned by the Vision API have no information in the Knowledge Graph?