It’s dependent on economics, politics, war, and immigration/refugees etc so very accurate predictions are limited to perhaps 6 months.
Total population has inertia, but anything more than +/- 2% out 10 years is pushing it and that uncertainty just grows with time.
Where are you getting the specific values of 6 months and ±2% out 10 years? Genuinely curious to learn more. Maybe back-testing of archived predictions from prior decades against the eventual reality?