Even though the Worldwide Harmonised Light Vehicle Test Procedure (WLTP) will provide a far more realistic representation of conditions encountered on the road than the old NEDC lab test (New European Driving Cycle), it will not cover all possible variations. Moreover, each individual driver will continue to have a different driving style: one driver might accelerate faster, take corners faster or brake more suddenly than another who might drive more conservatively.
Given that driving behaviour, traffic and weather conditions will continue to differ from one country to another, there will still be a difference between emissions measured in lab conditions and the real world. However, as there is no single real-world emission value, only values obtained by standardised laboratory tests allow us to directly compare the emissions and fuel consumption of different car models from different car manufacturers.