Comment 4 for bug 1257927

aptitude and apt-get are not equivalent. Aptitude remembers what you wanted to install or uninstall and it tries to get to that state again. So if you installed software with aptitude, but remove them by other means, aptitude may still remember that you previously wanted that software installed. (This is not a bug but a aptitude-only feature and you should not mix aptitude and apt-get for that reason unless you know what you do.)

For unattended installation or unattended package-management see aptitude-robot or pkgsync (which IIRC both use aptitude and not apt-get).