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.