When viewing stats in calendar mode, the daily average is calculated up to the current moment, not the end of the period. So it takes into account only the days of the period in which there could have possibly been a session. This can lead to changing and seemingly incorrect values as the current week progresses.
What we wanted to avoid was this, say that your goal is to meditate 20 minutes per day, if on the first day of a week you meditate for 20 minutes and then you look at the statistics and you see the daily average for the full week is 2 minutes and 51 seconds (20/7). It's kind of discouraging and in a way incorrect, as you have meditated 20 minutes per day in all the days in which it was possible to do so.
So instead it only takes into account the days which have passed up to and including today. So, for example, for the current week, on Monday it will take into account one day, on Friday five and only on Sunday all seven, giving 20 minute, four minute and 2m51s daily averages respectively, for that one 20 minute meditation.
For the previous weeks and months it should, of course, always take into account all days of the period in question.