The Active Days score is the highest number of days that you've been able to average that number of miles. So if you, at best, manage to average 25 miles per day for 25 consecutive days, then your score is 25! The idea of this is to reward consistency rather than just big miles, as your score will benefit from all those low mileage days when you're just popping to the shops or out for a countryside ramble.
So if I would have not ridden for 2 days and wanted to make a new activity, I needed to ride at least 30 30 30 = 90 miles to get the score for that period to 30.
I guess I understand :)
As for your score for previous years, I suspect that you only had 7 days in a row of that activity type. As soon as there's a day's gap in the activity, the score stops.