Most apps have some associated files that are not included in the app package. Typically, these may be small files such as preference lists (plist), but occasionally there are very large files that are not part of the .app package. The most effective method for complete app removal is manual deletion:
This is the method I use. My 2009 MBP has had quite a few pre-App Store programs installed and uninstalled, so I'm still finding random files years later. I don't touch anything that has Apple or something like that in the name. So far I haven't deleted anything important