If it were the case, the regulations would say "No more than X NOX emitted per mile, even when climbing Mount Everest, even when 5 years old, even when poorly maintained, and even when used with low quality fuel, and in all other conditions anyone might put them in."
That clearly isn't reasonable, so instead the rules are written to say "They must pass XYZ tests". As long as those tests are passed, they meet the standard.
IMO, the main culprit here is the lawmakers for not correctly designing the rules and test procedures to be sufficiently realistic. Specifically, they shouldn't allow the CO2 test and NOX tests to be done in different conditions.
They should instead say "your car will be assigned to one member of EPA staff every day for a month. They will drive their regular daily journeys with it, and all emissions recorded. If you aren't happy with the test results (for example due to a cold week), you may ask for a retest.".