It seems to also show Pull Requests in the Issues column. This might explain why, from the Github API docs:
Note: GitHub's REST API v3 considers every pull request an issue, but not every issue is a pull request. For this reason, "Issues" endpoints may return both issues and pull requests in the response. You can identify pull requests by the pull_request key.
Be aware that the id of a pull request returned from "Issues" endpoints will be an issue id. To find out the pull request id, use the "List pull requests" endpoint.
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