GDP growth figures published by e.g. the IMF are not just verbatim replications of what a country says. It comes from extensive analysis, verification, cross referencing against other data, and validation. It's why IMF data will often differ not only from what a country says, but even from what other organizations such as e.g. the World Bank or UN might say, as the latter are also independently carrying out their own similar processes. But all figures tend to fall pretty closely, because under this scrutiny it's practically impossible to meaningfully fudge things.