When you first filled the car it estimated your range based on your pervious driving, not the fuel you put in it - it has no way of knowing that.
The second time you filled it, the car still had no idea what fuel you had put in but again based its estimate on your previous driving i.e. when you drove on a full tank of premium. This gave you a higher range estimate - could be because the last tank was premium but could also be other factors such as less city driving, or a combination of both.