An engineer designs an improved automobile engine for a certain make of automobile. The previous design had an average highway petrol consumption of 10 l/km (litres per 100 kilometres) with a standard deviation of 0.5 l/km. The new engine has an average highway petrol consumption of 9.8 l/km, based on a sample of 3600 cars of the model with the new engine. Although the difference is quite small, the effect is statistically significant. The explanation is