It may seem counter intuitive, but this is how the Github page actually also shows it in the responses, which I found odd. After looking at this for a bit, this is my hypothesis: When you set "segmentable" filter on, it will only return items that match that. When it filter is left as default or set to false (which is the same), it will not filter at all instead of doing what you expect.
This to me looks the way it works, and if so, it a bit misleading.