Monthly Challenges

Why is it that the monthly challenge starts as one thing and then becomes another. This month I had a challenge to do more exercise minutes than last month, which meant averaging 89 minutes a day exercise. All was going fine, but on 11th November the 'Challenge' had changed to 'closing all 3 rings 0 [yes zero!] times' Which surprsingly I have completed!

This is by no means the first time I have seen the Challenge change in the first part of the month, but it doesn't do it every month. Is there anything I can do to stop this happening?