Geocode returning results outside of radius



I sometimes get strange results while searching for tweets with a geocode. For example, this endpoint searches tweets near Iceland:えろー&geocode=64.135666%2C-21.862675%2C500km

and it returns this tweet: which seems to be from Japan. I know that there is a fallback to the user location, but the user seems to be pretty far away from Iceland in this case. There are other examples like this if I search with the same geocode and different query.

Is the query wrong? What can I do to restrict the results only to the location I specified?