Wireshark-users: [Wireshark-users] Immediate ACK from server

From: vincent paul <amoteluro@xxxxxxxxx>
Date: Sat, 27 Mar 2010 19:22:11 -0700 (PDT)
Dear All,
I am looking at a trace between user and database server.  And I know for sure the RTT between them is 90 ms.
However, I observe that evertime user sends a request to server,  there is one immediate ACK from server to ack this packet (i.e. delta time between user's packet and its immediate ACK from the server is much less than RTT.  For example 0.2 ms compared to RTT of 90 ms).
Please explain how such server's immediate ACK could happen.