> The update request info you use to determine if it's been
> updated is commonly incorrect.

Commonly correct ?
>  However, on 99% of sites,
> they will return the correct size of the files. Could you
> put in another option to download based on size rather 
than
> using the server-stated modification date?
This is what "update hack" is supposed to do. But if the 
server does not give any valid content-length during header 
sequence, httrack won't be able to detect the freshness.

(To check that: activate 'Set options'/'Experts 
only'/'Activate debug mode' and check the hts-ioinfo.txt 
file)