Python-Twitter API's function, GetSearch, returns fewer results for a wider radius

Question

When I use GetSearch with the GeoCode, searching in a 1 mile radius returns the most results. This:

results = api.GetSearch(term = "treat", geocode = ("37.781157", "-122.398720", "1mi"), max_id = 1061378028763316224, count = 20)

returns 18 tweets, while this:

results = api.GetSearch(term = "treat", geocode = ("37.781157", "-122.398720", "5mi"), max_id = 1061378028763316224, count = 20)

returns zero.

Is there anything I'm doing wrong here?