If you have a DUT that is scanning for attacks, they may reset (RST) the connection on the attacking side in order to block an attack and on the internal side close the connection gracefully.
Since Avalanche considers any transaction that is RST part way through as a failed transaction the client side would be considered failed while the server would be successful and as such they would not match.
For confirmation, please run a validate test run and compare the pcap files.