Hello Samuel Befekadu Getu There are a few different sources for Ethiopian population data. As you noted, they are not the same. One of the main reasons for the differences is that a census has not been conducted in Ethiopia since 2007, and population data since then is based on projections.

An important source is the CSA, a government body providing official statistics as per the Government of Ethiopia: http://www.csa.gov.et/index.php You will find the census and projections on that website.

Another option is using UNFPA: https://www.unfpa.org/data/world-population/ET

For additional data, the World Bank Data Bank is quite useful: https://data.worldbank.org/country/ethiopia

Another good place for data is UNDP: http://hdr.undp.org/en/countries/profiles/ETH

In general, you should seek primary data or analyses using primary data in their projections (as opposed to secondary sources). If you wish to understand why data differs, that is a different, largely political and methodological question.

Thank you Logan Cochrane for your answer, I found it very clear and helpful.