Keep in mind the Volt’s energy usage screen is not tracking "miles using battery power" vs "miles when gas is being used." It is tracking "miles driven in Electric Mode" vs "miles driven in Extended Range Mode."
As you drive down a long hill, regenerative braking will likely put some charge back into your battery. At the bottom of that hill, your battery’s state of charge will likely be higher than it was when you started down that hill. Your Volt then travels a given number of "regen battery powered" miles using that regen, until the battery SOC drops down again to the level it was as you started down the hill.
Do you care if that distance is identified as "Electric" or as "Gas" miles? The label doesn’t change the nature of the travel... at the bottom of that hill, your Volt will be propelled by the electric motor using battery power put there by the regenerative braking system.
If you were driving in Electric Mode as you started down the hill, those "regen battery powered" miles will add to the "Electric" Miles you drove using the grid power you put into the battery when you recharged it from the wall.
If, on the other hand, your were in Hold Mode or had a fully depleted battery as you started down the hill, those "regen battery powered" miles will add to the "Gas" Miles you drove with the engine running.
Note that when you first switched into Hold Mode, the computer sets a flag marking that Hold point state of charge (it’s actually tracking the amount of usable grid power remaining in the battery). As you drive in Hold Mode, the SOC will fluctuate a small amount above and below that point (from normal operations, or from regenerative braking). If you switch back to Normal when the SOC is above the Hold point (for example, you exit the freeway, regen as you slow to a stop at the traffic light, putting your SOC above the Hold point, then switch to Normal for driving around town), the distance you drive on battery power until the SOC drops back down to the flagged Hold point will be recorded as Gas Miles. Often this is but a short distance and you don’t notice the small increase in "gas" miles.
It would appear that when you started driving down that hill, the SOC was above the Hold set point when you switched back to Normal. By driving down the hill, then continuing on using the regen obtained from the downhill drive, all performed with the SOC above the Hold set point, the entire distance was correctly recorded as Gas Miles.
Switching the Volt off and back on again when the SOC is above the Hold set point (or after using Mountain Mode to recharge a nearly depleted battery) will usually get a Volt to recognize the battery SOC is above the SOC of the remaining grid power, and will then count the use of this battery power as "electric" miles. The kWh Used number, however, should not change until the SOC drops to the remaining grid power level. This increases the total electric miles driven without increasing the kWh Used.
If your goal is to influence the electric or gas mileage by using Hold Mode to assign regen battery powered miles to one fuel category or the other, it can be done, but it doesn’t affect what really happens: so many miles using grid power from the battery, so many miles using regen battery power, and so many miles using gas to run the engine (and shouldn’t some of that, running on gas-generated electricity with no propulsion torque at all to the wheels provided by the engine, especially for a Gen 1 Volt, count as "electric" miles?).