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

Displays something like:

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