Sometimes packages are installed automatically on the system because some other package recommends it, but they are not actually dependencies of packages:
To know why an automatically installed package is present on the system, issue the command:
aptitude why package_name
result:
i aptitude Recommends apt-xapian-index i A apt-xapian-index Depends python-debian (>= 0.1.15)
In contrast, why-not finds a dependency chain leading to a conflict with the target package:
aptitude why-not package_name
result:
i ocaml-core Depends ocamlweb i A ocamlweb Depends tetex-extra i A texlive-latex-extra Conflicts textopo