Instruction 2 - 'Remove countries not reporting data from 1997-2017' - misled me a bit, and I was checking which countries had reported no data at all in that period using Summarize. My original workflow output similar but not identical results to the required Output.
I later went back for a hint, and saw that the solution workbook initially filters out countries that aren't reporting data in 1997, even if they then continue reporting data for other years (e.g. Burundi). Once I added this step in, I got the correct results!