De acordo com a página wiki oficial do Wireshark :
By default Wireshark and TShark will keep track of all TCP sessions and convert all Sequence Numbers (SEQ numbers) and Acknowledge Numbers (ACK Numbers) into relative numbers. This means that instead of displaying the real/absolute SEQ and ACK numbers in the display, Wireshark will display a SEQ and ACK number relative to the first seen segment for that conversation.
Mas, conforme explicado na mesma página, isso pode ser ajustado da seguinte maneira:
To disable relative sequence numbers and instead display them as the real absolute numbers, go to the TCP preferences and untick the box for relative sequence numbers.