Do I understand you correctly that you wonder how you can receive "faster that possible"?
The formula you state is correct. The window(s) and the RTT determine your bandwidth (there are other factors, but in most cases these are the important ones).
But I'm wondering about your numbers.
Ad 1) Are you sure about the RTT? This seems pretty high for regular downloads, unless the are transcontinental. Check the RTT by using ping (e.g. ping simtel.net
, replace the host name with the host name in question). You can use a more accurate ping utility like my hrping
(http://www.cfos.de/ping) (for Windows).
Ad 2) Are you sure about the Window size? 64k is pretty low today, all modern OSes try to negotiate more than that through RFC 1323 Window Scaling (http://en.wikipedia.org/wiki/TCP_window_scale_option). You can use SG TCP/IP Analyzer (http://www.speedguide.net/analyzer.php) to check your RWIN. Another greate tool for checking your connection is Netalyzr (http://netalyzr.icsi.berkeley.edu).
I would be interested to see the measured figures.