The payload hi is two bytes, and those two bytes advance the client count.
Payload bytes are counted
The payload is hex 0x6869, the two ASCII bytes for hi.
payload bytes=2
hi has length two
The byte h is one byte and i is one byte, so this payload length is 2.
Two bytes advance the count by two
Because the payload length is 2, the next client byte after this segment is two positions later.
1001+2=1003
Summary
TCP ACK arithmetic counts payload bytes, not messages.
bytes counted=2