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

Why does FortiGate keep TCP sessions in the session table for some seconds even after both sides (client and server) have terminated the session?
A . To remove the NAT operation.
B . To generate logs
C . To finish any inspection operations.
D . To allow for out-of-order packets that could arrive after the FIN/ACK packets.

Answer: D