Since this is "the % of our app's users using iOS6", the rate going down it probably means that in a given day the rate was 70% and the other less than that (me

Since this is "the % of our app's users using iOS6", the rate going down it probably means that in a given day the rate was 70% and the other less than that (meaning more total users, or just more active users that day with less than iOS6, etc).