Miscalculated TCP ACK ?

I'm having trouble with a particular server via HTTPS.  It appears
that my local linux machines are sending incorrect ACK.  However, I
don't have enough expertise to know for sure.

Using wireshark, the server sends:

    Transmission Control Protocol, Src Port: https (443), Dst Port:
36015 (36015), Seq: 27658, Ack: 827, Len: 18

Local machine replies:

    Transmission Control Protocol, Src Port: 36015 (36015), Dst Port:
https (443), Seq: 827, Ack: 27677, Len: 0

It appears to me that the ACK is off-by-one (should have been 27676).
The server replies again with retransmissions.  This is with several
kernel versions on local machines, ranging from 2.6.31 to 2.6.38.

Can anyone shed insight into what's happening?  This looks like a bug to me.

Thanks,
Gabriel