Why doesn't regen post miles to my EV capacity when it is zeroed out.

With our 2016 Volt I have noticed when my wife and I take a trip down the coast, 130 miles or so total, and 60 miles or so on gas, when I arrive a few miles from home, I usually have a few miles of electric left and then with the battery exhausted and 0 range indicated I'm still running on electricity for a mile or so and the computer is adding them as gas miles. Just seems strange like those electrons just appeared preventing the gas motor from starting up. No complaints last fishing trip we ended that trip with 60.2 mpg on gas, 57.9 miles, and nearly 70 miles on electric. That is about as good as it would get for our 2010 Prius for gas mpg's.