The energy usage screen displays Electric Mode vs Extended Range Mode numbers, not battery miles vs ICE is running miles. Electric Miles numbers identify distances driven on the grid power from the wall stored in the battery, and on any regen put back into the battery while driving in Electric Mode. The kWh Used number is a net calculation of the grid power used less regen put back into the battery.
Regen works just the same when you’re in Extended Range Mode (Hold, Mountain Mode, fully depleted battery), giving you battery powered distances accounted for under the Gas Miles display.
Counting these regen battery miles as Electric Miles would give a distorted picture of how far your Volt can travel on the amount of usable grid power stored in the fully charged battery when compared to how far an all-electric car could travel on the same amount of usable grid power stored in their battery.
The function of the braking regeneration system, of course, is to serve as a method of slowing/stopping the car. Recharging the battery while braking the car is a bonus benefit.
As you might know, when the Gen 1 Volt is in Extended Range Mode (Hold, Mountain Mode, or fully depleted), the gas engine’s shaft is clutched to the smaller electric motor’s shaft, operating it as a generator, creating electric fuel for the primary electric motor. In one sense, 100 % of the Gen 1's miles are "electric" miles.