Not true. Emissions vary a lot depending on environmental factors and optimizing for the worst case scenario (which is what "within limits at all times" implies

Not true. Emissions vary a lot depending on environmental factors and optimizing for the worst case scenario (which is what "within limits at all times" implies) would likely result terrible averages. Even within the test cycle, the engine can be as dirty as they like at any point in time, as long as the average stays within bounds. What they are not allowed to do is change the way the engine operates when under test vs on the road, and this is exactly what happened at Volkswagen. The VW case is not the first (ruling like this tends to be reactive, and slow...), but it might be the first were the response is firm enough to deter future would be offenders.