What you are seeing is the new method of calorie calculation, but you are able to switch to the older method if you would prefer. With the newer method, your calorie range will adjust when you track fitness activities or your Fitbit syncs your steps/distance info. So yes, if the range goes up, the lower half would become the new minimum for the day.
For more info about the two methods and figuring out which one might be best for you, please see this page: www.sparkpeople.com/community/help_answer.
Hope that helps!