I’m currently on a 5-day streak (at least according to the calendar), but at the bottom of the calendar it says it’s 9 days long.
I suspect it’s remembering and adding my streak from the free trial several months ago. That, or I’m misunderstanding the number, and it’s counting total days studied regardless of whether they were consecutive…but if that’s the case, having it beneath the “study streak” tracker is a bit confusing.