It seems like the reason your results differ from the World Bank is that you're taking German GDP per capita figures translated into dollars according to the ex

It seems like the reason your results differ from the World Bank is that you're taking German GDP per capita figures translated into dollars according to the exchange rate at the time without adjusting for purchasing power, and then you use US consumer inflation to compare them across time. Whereas the World Bank appears to derive its figures by applying the German inflation rate (which was lower for most of the 30-year period) to make GDP per capita comparable across time, and then translates the values into US dollars while compensating for purchasing power differences. If you earn income in Germany but spend it in the US, the former scenario would be quite relevant, but under normal conditions I think the World Bank's calculation is more useful. It's interesting though that the two ways to compare the value of money over time and space lead to such different results.