For demonstration purposes, take: 
      <http://www.cygwin.com/setup.exe>

Chose Download Site
In the middle of the download Press "Skip"
Start again using "Continue interupted download"

Alpha-3 starts in the middle and downloads only half of it. 
Alpha-7 starts from the beginning

I have switched on -%H and have two hts-ioinfo.txt files.

What I spotted as main difference in the GET part is:

Alpha-3
*************************
[1] request for www.cygwin.com/setup.exe:
<<< GET /setup.exe HTTP/1.1
<<< If-Unmodified-Since: Sun, 01 Feb 2004 19:32:32 GMT
<<< Range: bytes=114394-
<<< Connection: Keep-Alive

Alpha-7
*************************
[1] request for www.cygwin.com/setup.exe:
<<< GET /setup.exe HTTP/1.0
<<< Connection: close

The response is a 206 in case 1 and a 200 in case 2 (plus 
different byte counts and Keep-Alive parameters)

I can post the full hts-ioinfo.txt files or other infos if 
it is necessary.

Best regards 
   Juergen