It doesn't happen that wayThis can be an indication of one bad cell group.
Change that to 3+ yearsYes but it could take a year to get a new battery and I would rather start with a car that is not already having an obvious problem that increases it's risk of a fire.
Pack voltage at max range in a normal pack is around 400 volts. A weak cell group of the sort that GM hoped would indicate an at risk cell for fire may be 5 - 50 milliVolts lower than the average cell when fully charged.If the car has one cell group with less capacity than the rest, the maximum range will be reduced.