So I actually have the exact same JuiceBox charger as the OP, and have seen the exact same issue with my Niro EV. I've used the JuiceBox with several other EVs over the years and I've also written some custom software to control it and pull data from it, so I've learned a lot of it's nuances over the years.
One of the "features" of the JuiceBox is being able to control how much charge is delivered to your car. For example if you have an EV like the LEAF which doesn't allow you to specify charging should stop at 80%, you can do this via the JuiceBox instead. Let's say you get home at 20%, plug in and you want the JuiceBox to charge to your car to 80%: The JuiceBox asks you to set what charge you plugged in at, and then asks you what charge to stop at. Remember that the HJuiceBox cannot communicate with your car in any way, so it's using these values to "estimate" how much energy to deliver.
When you set up the JuiceBox you had to tell it what EV you were setting up. When you selected the Niro EV you got a screen that looks like this:
Notice that it has fields for "Battery capacity" and "EVSE Efficiency"? So when you set up the car it comes pre-populated with data about the car: That it has a 64 kWh battery and the charging system is about 98% efficient.
Now that it knows this, when you plug in you get a screen like this:
See the 20 selected on the left, and the 80 selected on the right? Here I've told it that I "plugged in" with 20% and that I want to charge "up to 80%"
So what happens here is that since the JuiceBox can't actually communicate with the vehicle to know once it hits 80% it has to do the math to calculate how much energy is needed to get to 80%.
So if you plugged in with 20%, and you only want to go to 80%, then that means we need to add 80-20=60%. Since we need to add 60% of a 64 kWh battery that means we need to add 60% of 64 kWh = 38.4 kWh. Then it factors in the 98% efficiency and pulls 39.18 kWh from the wall. Within the app however it only displays 38.4. The amount displayed in the app always factors in the 98%, but it actually pulls 39.18 from the wall.
Now what happened in your case is you left it at "0%" on the left and "100%" on the right. What this told the JuiceBox is that you wanted to add 100% of 64 kWh, which is 64 kWh: so it stopped charging after it delivered 64kWh. The thing about the Niro is it actually has a slightly larger than 64kWh battery pack. (This has been confirmed by multiple people). So even though it's officially rated at 64kWh they often have over 70 kWh usable capacity when brand new. Some speculate Kia does this so that when the pack degrades over the first year or so it never drops below the "rated" capacity, but who knows.
So to sum it up: it's not due to efficiency loss. That's already being factored in by the JuiceBox. When I charge my LEAF I actually see the opposite: When the Juicebox delivers 60% to the LEAF it actally adds more like 70% according to the car. That's because Nissan claims a 62 kWh battery but if you check LeafSpy you'll often see only 57-58 kWh is usable, especially after you've put some miles on the car and the battery degrades a bit. If I go into the JuiceBox and "edit" the capacity of my LEAF and set it to the same 57 kWh that LeafSpy says it holds, then suddenly the percentages of the JuiceBox match up perfectly with the percentage reported by the car.
So in your case the fix is to go in and edit that field. Where is says the capacity is "64.0 kWh" set that to what your actual capacity is. Since adding 64 kWh to your battery took you from 2% to 92%, that means it added 90%. That puts your actual usable capacity at 71 kWh. (My Niro is brand new and has about 73 kWh usable energy)
Further evidence of this can be seen when planning routes with "A better route planner" (ABRP). If ABRP tells me I need to charge to 75%, I've found I only need to actually charge to 65% because there's an "extra 10%" of usable capacity available in the battery. With my LEAF I have to charge a little beyond what ABRP suggests, but with the Niro I can easily cut 10% off what it says and still make it to the next charger with energy to spare. (While driving 5-7 mph over the speed limit too)
Bottom line: The Niro EV actually has a slightly bigger battery than advertised (Which is awesome for road trips) so you just need to adjust the capacity on the JuiceBox to reflect that.