Opps, editing too long and now a late answer
100% amounts to a certain amount of electrical energy. It takes more energy to push the car at higher speeds (air resistance becomes much greater with speed), so it takes more energy to cover 10 miles at 70 than it does to cover 10 miles at 20 or 40. The car is making an estimate, a guess, on how far it will be able to go based on your recent journeys. I don't think any of us know precisely how many days or miles are taken into consideration and whether recent miles are weighted more strongly than older miles...
If you have a pattern of use of the car which takes you on a daily commute at high speed it will give a different estimate for how far a 100% charge will take you than if you commonly do low speed school runs and urban driving. It will have learnt (on the basis of recent use) how many miles per kilowatt-hour you are typically getting. It is revising this decision all the time.
For instance consider this easily imagined scenario: a driver spends a month pootling around locally at low speed, then at the end of the month the driver takes a rare journey from London to Edinburgh (400 miles at high speed). After the period of low-speed driving the car has estimated 4.5 miles/kWh as your rolling-average, so the 64kWh battery can be estimated to give you 288 miles of range. The driver plans a stop for recharging and something to eat at about 260 miles, hoping that the 28 miles contingency is enough. As the journey progresses the car will be likely to be performing in the range 3-4 miles/kWh and it will be adjusting its "guess" for remaining distance accordingly. The driver may be forced to stop earlier than expected. Over the course of the journey the range indicated will have been correctly predicting the shorter range. At the end of the journey the driver plugs in at the hotel - In the morning the car reports 100% and 230 miles of range, because that is a reasonable guess based on recent performance. People often refer to this as the "guessometer". It is a bit unfair because it is actually quite accurate at estimating what you need to know, most of the time. Many reviewers have been complimentary that Kias are quite good at delivering the range that they indicate (more so than some other brands). The guess can be "off" when the pattern of use changes.
So you new e-Niro may have been delivered to the dealership and to you with only a few low-speed short journeys. As it is taking more recent journeys into account it is adopting a lower miles-per-killowatt-hour figure. You said "??? I didn’t the heating in or anything ??"... You are right to consider the heating, it is a factor, but I suggest that your pattern of use and the local temperatures have altered the efficiency.
Personally I like to have the "Trip" information shown on my dash display, there are various display choices. The "Consumption" display is a bit too instantaneous for my liking. I prefer the "Drive Info" which shows the rolling average of this "drive" since you started. If you stop for less than 4 hours it considers the next drive as just continuation of the same "drive". Using this I can often see that the efficiency starts low at around 3.0 miles/kWh (when the heating is warming the cold car), and the number can rise to more like 3.8 as a journey progresses over 10-30 minutes and the heating has stopped doing as much work. I also find the "Drive Info" useful because it means that I have a good idea how a particular journey is going. In mild weather I expect the efficiency to be over 4 or at least high 3.x values. For a really cold start I expect around 3.0, but it can be way better by leaving the heat off. I have the heat on because the "hit" isn't as great once the car has warmed up a bit.
[An aside: The "consumption unit can be expressed as miles/kWh or kWh per 100 miles. I find the miles/kWh to be handy small numbers roughly in the range 3-4, where less than 3 is bad and >4 is doing very well. I find the how many kilowatts to do a distance a bit odd - we all talked about miles-per-gallon mpg, not gallons per hundred miles!]
