The most common reason is that some uninstallers, in the process of doing their job, will themselves create and leave folders on the system. Others will still leave some harmless folders belonging to the antivirus behind. Anyway, it is always a good practice to run the uninstaller in Windows safe mode and to restart the computer after it finishes, to assure the best results.
Because the purpose of the functionality is to inform you about what products were installed in the past, so that you can run the specialized uninstaller. The specialized uninstaller will do a proper job removing all leftovers (files, drivers, services, registry entries) and not only folders.
Rarely, but yes, it is possible. Always expand the nodes to verify and confirm the results. Note that the program will simply present the detected folders, it will not delete or perform any other operation on them.
The specialized uninstallers are taken from the official sites without modifications and are provided “as is”. Always make sure to follow the tools instructions. If you find problems, please contact the utility manufacturer.
All the ones listed in the combobox of the program (29 in total). Those are the most common and the ones that provide dedicated uninstallers. Here’s a list: Adaware, Avast, AVG, Avira, BigFix, Bitdefender, Comodo, Dr.Web, Emsisoft, eScan, ESET, F-Secure, G DATA, K7 Computing, Kaspersky, Malwarebytes, Max Secure, McAfee, Microsoft Security Essentials, Norton, Panda, Symantec Endpoint Protection, TrendMicro, Total Defense, TrustPort, Vipre, Webroot, WinPatrol, ZoneAlarm.