More than likely, assuming your measurements are correct, there is a local "blackbox" between user and the server. This will possibly be an old-school application proxy (or a firewall acting as such a proxy), a device like Packeteer doing traffic-shaping, or a new-age WAN acceleration device (such as from Riverbed, or a Juniper WX or Cisco WAAS).
These all can fake the ACK, and do so simply to either avoid the problems of delay on WAN traffic, either trying to serve cached traffic or manage the sliding Window to improve (or hinder) your throughput.
Regards, Martin
MartinVisser99@xxxxxxxxx