People can agree to go to an event but have very different feelings about doing it. Some people will be extremely stoked about going, some will be happy about it, some will be like meh its better than sitting at home, some people won't want to go but decide to anyway because they want to be nice, some may really not want to go but decide to go anyway out of a sense of obligation. The particular words you reply with express your interest level to some degree. A plain "k" will be interpreted as a meh at best.