These differences arise because the species population indices at national and supranational levels are recalculated each year again for the full-time series. As national schemes increase their capacities to analyze old data (e.g. data not computerized before), new data from the early years become available in some cases, which can influence the index values. When index values change, the multispecies indicator values also change. However, these changes are small, and they are of minor importance because the interpretation of indicator values should focus on long-term trends.
Learn more in Methods, chapter 2: Supranational species indices and trends.
Read more about the interpretation of indicators in the next question, “What exactly does the multispecies bird indicator tell us about changes in bird diversity?“