Packets can arrive out of order from the Internet Protocal layer. So,
when out-of-order packets with higher-than-expected sequence number arrives, then gap is detected.
Receiver of an out of order packet would then generate and sends a duplicate acknowledgement (ACK) for already received packet. If retransmission is performed after the first duplicate ACK, it would lead the sender to introduced too many redundant packets in the network.
Hence, TCP designer chose to wait for third acknowledgement (ACK) instead of performing a fast retransmission to avoid redundant packet transmission.
