Because the interpretation of this report is that if a person did not know about the occurrence of impurity at the time of the prayer and then discovered it afterward, it is not obligatory for him to repeat the prayer. As for the first report, it concerns one who knew about the impurity on his garment and did not wash it, whether deliberately or out of forgetfulness, thus he must repeat the prayer afterward. We have fully detailed this in the Book of Purity and have cited therein the relevant narrations, including the report of Zurarah and others. What further clarifies this is what has been narrated:
Commentary #4