An assumption required to reproduce inclusive hadronic cross section data

For 50 years, Standard Model calculations have successfully reproduced inclusive hadronic cross section data from high-energy $e^+e^-$ collisions. However, these calculations have always assumed that there is no ``flavor interference'' between $e^+e^-\to u\bar{u}\to X$ and $e^+e^-\to d\bar{d}\to X$ events (where $X$ is the same hadronic state). That assumption is questioned.