Why does FortiGate keep TCP sessions in the session table for several seconds, even after both sides (client and server) have terminated the session?

Why does FortiGate keep TCP sessions in the session table for several seconds, even after both sides (client and server) have terminated the session?

A. To allow for out-of-order packets that could arrive after the FIN/ACK packets B. To finish any inspection operations C. To generate logs D. To remove the NAT operation