Geocode returning results outside of radius

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geo
api

#1

I sometimes get strange results while searching for tweets with a geocode. For example, this endpoint searches tweets near Iceland:

https://api.twitter.com/1.1/search/tweets.json?q=えろー&geocode=64.135666%2C-21.862675%2C500km

and it returns this tweet: https://twitter.com/fran723_ka/status/841239319687790592 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?