run 1 - site downloaded

run 2 - site updated. old cache renamed "old.zip". "new.zip" contains only
updated files

run 3 - site updated again. cache from run 2 renamed "old.zip". files updated
in run 2 are checked for updates. files not updated in run 2 are no longer in
the cache, are assumed to be new, and get downloaded again.

is this really what's happening? or am I missing something?