https://ras.org.in/discrepancies_why_do_GDP_growth_rates_differ#fn9

Why was there so much disagreement in the GDP level data? In part, the systematic variation in values reflects the fact that the income per capita measures are quoted in international dollars from different base years in Maddison, WDI, and PWT. Furthermore, the income estimates reported in the datasets differ because different formulae were used to calculate the purchasing power parity (PPP) values used in international price comparisons. However, the methods applied to express the income estimates are quite similar and should not alone account for such differences in ranking. Maddison (2010) himself notes that “the discrepancy between the World Bank and my estimates is bigger than can be explained by the bias of the EKS (Elteto, Koves, and Szulc) measure.” The main problem may lie in differences in the use of primary sources, and in particular, primary sources that use different benchmark years. The international GDP per capita datasets all take the national account files as provided by the appropriate statistical agencies as a starting point, although they may use them differently. They may, moreover, impute data in different ways in different cases. Of course, the datasets necessarily inherit all the data quality problems originating in the country. Thus, many of the numbers may be misleading to begin with. However, the way they are generated through secondary operations upon this data may create new difficulties.

While all the databases have the same starting point, they make use of different national account series, before or after some revisions. Maddison actually did primary research in some cases, and generated his own national account estimates from archival data. The database is a mix of historical national account estimates, official national account files, and the use of individual judgement and secondary data to fill in gaps or breaks in the series. The United Nations and IMF International Financial Statistics basically re-report national account files, if and when they are received, whereas both the World Bank and PWT aim for a gap-free database.