With older technology the best you could do was to figure out a setting which would allow an engine to perform across a range, for race tuning for a track for example you would be looking for power at higher speeds and not for sitting in traffic idling.
A fully loaded car will require more torque at lower speeds than one carrying just the driver.
Going uphill requires more power than driving on the level. On older vehicles pretty much the only thing that could be done was to lower the gear and increase the revs.
Put simply there was one tuning setting which had to cope with all driving conditions. What Volkswagen have done is make the tuning variable on the fly and the engine management provide the optimal tuning for the conditions, rather than using a compromise tune.
Unfortunately this means that for a rolling road on the level where there isn't as much need for power the engine reduces its fuel intake and burns an appropriate amount of fuel for the conditions it detects. The problem isn't with the car so much as the actual emissions testing methods.
Think about it when your driving on the level and come to a hill you will press harder on the pedal to dump more fuel into the engine some of which will pass through unburnt. If you see a diesel engine vehicle in front, you often see a puff of smoke as the driver dumps more fuel into the engine. usually its for around a second as the vehicle picks up speed. In that second the emissions wouldn't pass inspection.
The emissions test is supposed to be representative of typical engine output with volkswagen engines the output under test is what you would get in similar driving on the road, unfortunately real road conditions are not driving with low resistance at a steady speed.
This is a bit of a disaster for new cars because if they are not allowed to perform optimally then the end result will be higher emissions in order not to be cheating a defective test.