Question

Why May looking at only data from the national level not give an accurate understanding of what economic conditions in a country may be

Answer 1

data from the national level does not give an accurate understanding of what economic conditions in a country may be because national records might show growth or improvement but due to inequality, most or a larger part of the country's population might fall below this national record level.

A better look into a country should be based on the Inequality Human Development Index IHDI which shows the distribution of a country's wealth factored on inequality within its population.