Angola has a larger economy and thereby a larger per capita income than Kenya, far much larger. But per capita income is only one metric used in determination of how developed a country is. And it is largely flawed for mineral dependent nations, Angola being a classical example of such. To factor in all aspects affecting the well being of a country, you use the Human Development Index
The HDI is a composite statistic of Health (life expectancy, access to health services) , education (years spent in school, % of literacy) , and Economic (per capita income, access to basic infrastructure like paved roads and electricity) indicators, which are used to rank countries into four tiers of human development:-
1. Very high HDI (developed nation)
2. High HDI (NICs and emerging Nations)
3. Medium HDI (Frontier and Developing nations)
4. Low HDI (LDC)
Angola is at tier 4, same as Tz while Kenya is at tier 3 so the grouping ain't off.