Agreed, but the car doesn't provide you with capacity, 100% range is all we have easy access to and it's a way to tell which battery you have (e.g. a 75 kWh or a 79 kWh battery in an LR), as well as providing an indication into battery degradation over time.Using max range to compare battery capacity is wrong, battery capacity should be used to compare battery capacity.
Tesla has been using a simple kWh per mile constant to calculate the 100% range, so two people with the same model/battery type are effectively comparing battery capacity.