That happens when a previous drill question you answered is ready to be answered again. The points from that question come off your total score if it's worth more points than the one you just answered.
Next time it happens, look at the number by the "Now" under Scheduled Drills and you'll notice it doesn't decrease even though you just answered a question.
Makes sense. I had not considered that. Thank you!lesboys22
Commented 6 years ago