> Is it because I have to tick 'Do not purge old files'?
There are two issues:

- If the option 'Do not purge old files' is not checked, 
files not anymore on the server will be wiped
- When updating, httrack asks the server for file 
freshness. If the server deems the files not up-to-date, 
the file is transfered again to get a newer copy.

Some servers are bogus, and will always badly repond to 
update requests, leading to useless redownloads.

But aside from that, the update process should be fine.