Each report slices different data differently;
- Experian report doesn't include households with no cars (and doesn't include other personal vehicles like motorcycles)
- NHTS report includes all vehicles, even golf carts and commercial vehicles, but only if the contacted household (which was reachable by land-line; no cell phone survey results were included) agreed to participate in the survey
- Carnegie report looks at raw numbers of registered passenger cars & light trucks (no commercial vehicles and no other personal vehicles) per capita (rather than per household), thereby including those that cannot drive yet. Using US Census #s of an average of 2.6 people per household, it comes to 1.27 passenger cars per household
How is it relevant? I would say that the (cell phone reachable only) young people will buy cars later (than we did when we were young) and when they do, it will need to meet all their (long or short distance traveling) needs.