Keep two things in mind:
1. It’s the same motor/generator under the hood that both pushes the car and creates the regen, and you can’t do both at the same time.
If your foot is on the accelerator as you head down the hill, either to maintain control because the road is very curvy. or the slope is not steep enough for gravity alone to pull the car down the hill at the desired speed, you’re not getting any regen at the same time, and overall downhill regen is less. With the foot on the accelerator, the motor provides propulsion torque to the wheels via the drivetrain. With the foot "off the gas" (so to speak), the car’s momentum turns the wheels, which, via the drivetrain, can use the motor as a generator. The computer can adjust the electric circuits to adjust the rate of generation, which changes the amount of the car’s momentum being used to turn the generator. The greater the regen rate, the faster the loss of momentum (think regen in D vs L or paddle vs D).
2. The computer creates an "on the fly" ev range estimate as you drive. Downhill driving is more fuel-efficient than level or uphill driving. The increased range estimate at the bottom of the hill is, in part, because your computer thinks you are going to keep driving downhill, and so can travel further on the remaining amount of power. It not necessarily because the downhill regen added more fuel into to the battery than was used during the drive down the hill.
For example, in good weather, my ev range estimate may increase from 40 to 41 miles by the time I reach the grocery store 4 miles from home that is at a lower elevation. My usage screen tells me I’ve actually used a net 0.4 kWh of power to get there...