The difference between Jimmy Neutron and Cindys' relationship to Lincoln and Ronnie Annes' is that lincoln and ronnie anne became friends very early in the show and keeping their dating life a secret because lincoln doesn't want to hear kids teasing him about it and ronnie anne wanting to keep up her reputation. Jimmy and Cindy on the other hand while they did act as allies sometimes and are seen with the others they still held grudges against each other through most of the show and didn't start showing more interest in being friends then lovers until like season 2 or 3 or whatever.