Usually thats a result of a couple things.
Some left-behind files belong to the program. Which is an issue with the programmer.
Some left-behind items are files created by your useage of the program.
Some get left behind due to an upgrade.
Many files information is also left behind in a file called index.dat.
There are many program which exist to remove these stragglers
http://www.softpedia.com/get/Tweak/U...-Manager.shtml