I don't believe the Niro battery charge indicator has anything to do with a 100% capacity range. Rather, it is likely indicating a charge of one third at the bottom end of the scale, and two thirds (perhaps 80%) at the top of the scale. Niro has aggressive battery management to restrict charge in the middle third of capacity to increase longevity of the battery.
On the virtually identical Hyundai Ioniq drivetrain, but with a different user indicator system it shows charge almost invariably kept within that range. The ICE will fire up if it drops below that range. There is one scenario that has been reported where you can fully charge the battery, and that is on a long extended downhill where regenerative braking is active. Even there, if the battery charge level gets too high, the ICE will spin up for compression braking so the battery doesn't overcharge.
There are lots of posts on Ioniq forums from puzzled new Ioniq owners wondering why the battery is never charged fully, because of the way their indicator works. You can get some more accurate depiction from OBD2 tools but I'm not sure how that benefits hybrid owners. EV owners for sure. I was watching a Tesla Model 3 drive (review) last night, and they discussed different setting of the battery management software for different driving conditions. Obviously, on a long road trip, you want full use of the maximum battery capacity and charge fully before leaving and at superchargers. Twenty to fifty miles a day around town and you have no reason to do this. You'd rather ensure maximum battery life by keeping it in the sweet spot for the chemistry of the batteries you are using, usually somewhere between 40 and 80 percent for most lithium battery variations.