These two phenomena are not necessarily the same. Repetition can be a good thing. Sometimes we have to use our key terms several times within a paper, especially in topic sentences. Sometimes there is simply no substitute for the key terms, and selecting a weaker term as a synonym can do more harm than good. Repeating key terms emphasizes important points and signals to the reader that the argument is still being supported. This kind of repetition can give your paper cohesion and is done by conscious choice.