On "Survivor", why was Marty not voted off?

The votes were split between Marty and Kelly B., but I don't understand why the tribe didn't vote Marty and his immunity idol out. It seemed like everyone but Jill was onboard to take him out (including Fabio), but they voted 5-1 the second time to get rid of Kelly B. This is my first season watching so maybe someone can explain that to me.