Not collected all tweets?

Hi all,
I have a question how to collect all the tweets for one city using this code.

In my experience, it seems like not collected all the tweet. For example, I executed this command (python2 Exporter.py --near "cityname, England" --within 15mi --since 2018-08-30 --until 2018-08-31) to collect tweet created on 30 August 2018 (only one day) for certain city. It returned approximately 71 thousand tweets for one day.

But, when I tried to collect tweets created on 1 June 2018, only returned 134 tweets. In my opinion, it should be more than 134 tweets for one day (probably more than 10 thousand tweets). How come this is possible? Is anyone have a solution for this? thank you.