Because the averages are skewed because in a countries population it is safe to assume that a certain amount of people will be unemployed or low income earners. Immigrants to a country nearly always have full employment so of course it would show the average per person is higher, because the two statistics are not created equally. Also, a lot of immigrants tend to take up skilled employment as doctors, financial advisors, IT specialists, etc. and those jobs always have a higher salary so of course more tax is paid. It's like saying the average height of immigrants is higher than the average height of nationals, it might be true but the samples to create the statistic are heavily influenced by a higher number of factors for nationals than for immigrants.