I think you have to tell HTTrack to go look at the site so it can tell if
something's new.

I found this in the online Manual:

Q: I want to update a mirrored project, but HTTrack is retransfering all
pages. What's
going on?
A: First, HTTrack always rescan all local pages to reconstitute the website
structure, and
it can take some time. Then, it asks the server if the files that are stored
locally are
up-to-date. On most sites, pages are not updated frequently, and the update
process is
fast. But some sites have dynamically-generated pages that are considered as
"newer" than
the local ones.. even if there are identical! Unfortunately, there is no
possibility to
avoid this problem, which is strongly linked with the server abilities.