Some people forget that regen happens in any driving mode, and that your Volt may be traveling on regen battery power even when the battery is fully depleted or you’re in Hold mode.
Are you sure you want to itemize your regen? If the usage display shows you drove your Gen 2 Volt 53 Electric Miles using the 14.1 kWh of grid power in the battery, and the regen display shows you obtained 3.9 kWh of regen power during the drive, then your ev mileage for the drive is actually 2.9 miles/kWh, not 3.8 (53/18 vs 53/14.1).
Increasing the amount of fuel used to drive a given distance will always lower the mileage. The increase in total useable power provided by regen during a trip does not necessarily mean you can drive further - sometimes the difference in terrain that causes the increase in total regen also reduces the overall ev mileage (e.g., regen from downhill driving offset by uphill driving mileage reduction).