Well a lot of secondary research is, in fact, expressing someone else's ideas. It takes a lot to come up with truly original ideas. It does seem a bit unfair that references and discussion of those count as "hits".

On the other hand if two documents share more than a few phrases, it does seem to be dancing into plagiarism. Maybe. Some phrases are of course attached to a topic and difficult to avoid. I suppose that is where a teacher needs to be attentive and reasonable. It is the sloppy hand-ins that are virtually identical to others that need to be caught. Although, it sometimes seems that even getting caught makes little difference.