Entries in the Add/Remove programs list are optional and are put there by software vendors that choose to do so.
Some software is smart enough to know about their other softwares installed and can control them from a single add/remove slot. When this is the case usually when you click the remove button you will get a window with more options. Like, remove- reinstall- and modify are typical.
It sounds like this may be what you are seeing. Also there is often an uninstall entry in the programs folder or start menu group.
Info about add/remove How to change or remove a program in Windows XP