Why does Aptitude think that packages purged with dpkg need to be reinstalled?

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http://ubuntuforums.org – I have noticed using Aptitude to apply updates, and dpkg -P to purge off old kernel versions, that after purging old kernel versions, Aptitude wants to reinstall the old kernel packages next time I am in there to apply updates. As is, I must tell Aptitude to forget pending changes / available updates, re-download the list of packages, mark all upgradable, THEN it correctly marks only the upgrades and does not offer to reinstall old kernel packages which I have purged off with dpkg -P. Why do Aptitude / dpkg not keep better track of what packages I do / do not want installed? TIA! (Hardware)