--mtime gets some older files too

I notice --mtime does not work perfectly. Some older files get included.
Here I had used
mknmz-wwwoffle -f search/namazu/conf/mknmzrc -T search/namazu/conf -O
search/namazu/db -V --no-delete --mtime=-16 http ftp

which calls
/usr/bin/perl -w /usr/bin/mknmz --allow=D.*$
--replace=wwwofflecache::replaceCode --no-encode-uri
--target-list=/tmp/mknmz-wwwoffle-target-list.1674 --mtime=-16
--output-dir=search/namazu/db --template-dir=search/namazu/conf
--verbose --no-delete --config=search/namazu/conf/mknmzrc

but I noticed it was processing some older files, like
$ stat /var/cache/wwwoffle/http/www.ris.gov.tw/?q+uuYWg+JosG4U3wgyCScQ
  File: `/var/cache/wwwoffle/http/www.ris.gov.tw/Dq+uuYWg+JosG4U3wgyCScQ'
Modify: 2001-05-13
Change: 2004-01-31

  File: `/var/cache/wwwoffle/http/www.ris.gov.tw/Uq+uuYWg+JosG4U3wgyCScQ'
Modify: 2001-05-13
Change: 2004-01-31
$ wwwoffle -O $(cat /var/cache/wwwoffle/http/www.ris.gov.tw/Uq+uuYWg+JosG4U3wgyCScQ)|head
Date: Sun, 13 May 2001 11:30:05 GMT