TCP Flags Explained – What Do TCP:RA, TCP:FA, TCP:PA, TCP:S, TCP:SEC Mean?

http://en.wikipedia.org/wiki/Transmission_Control_Protocol says

Acknowledgment number (32 bits) – if the ACK flag is set then the value of this field is the next sequence number that the receiver is expecting. This acknowledges receipt of all prior bytes (if any). The first ACK sent by each end acknowledges the other end's initial sequence number itself, but no data.

This suggests that the first reply packet is acknowledging receipt of the first three packets from vendor (seq 76905 + len 1440 = 78345)

The second reply packet acknowledges receipt of the 4th and 5th packets from vendor (seq 79785 + len 233 = 80018)

The third reply packet indicates no futher data has been received from vendor (same ack) and contains a payload of 197 bytes.

This looks OK to me.

If your data is the whole conversation, the initial ack would be wrong as it should ack the first packet from vendor with an ack of 75465 (seq 75465 + 1 = 75466).

Here is a sequence I captured with wireshark, First we see the three-way handshake, then the transmission of a HTTP get request followed by an HTTP response

Source    Flags      Seq      Ack     Len  
client    SYN          0        -       0  
server    SYN,ACK      0        1       0  
client    ACK          1        1       0  
client    -            1        1     429     Get ...
server    ACK          1      430       0  
server    -            1      430    1456     HTML response
server    -         1457      430    1456  
client    ACK        430     2913       0  
...   

The sequence and ack numbers are relative (to each end's randomly chosen starting number)

Using a single connection for a sequence of requests is a common optimisation. It was added to HTTP at version 1.1 and known as persistent connections. Setting up and tearing down TCP connections has a cost.

This:

-p tcp --tcp-flags SYN,ACK,FIN,RST SYN -j DROP

means "look at the flags syn, ack, fin and rst and match the packets that have the flag SYN set, and all the other unset"

The first argument of tcp-flags is the flags your are considering, the second argument is the mask you want to match. if you were using

--tcp-flags SYN,ACK SYN

then it would match packets that have [SYN=1 ACK=0], but it would not match packets that have [SYN=1,ACK=1] or [SYN=0,ACK=1] or [SYN=0,ACK=0]

In your rule above, you are matching SYN packets only.