It is quite likely that the reported GDP (and consequently
GDP per capita) measures for poor countries significantly understate the extent
of economic activities actually taking place.
Morten Jervens on the challenges of GDP measurement in Africa
Bill Gates highlights the challenges posed by
mismeasurement of GDP:
http://www.project-syndicate.org/commentary/poor-countries-need-more-accurate-gdp-data-by-bill-gates