algorithmic problem?

Is it right to recommend only p(roduct)4, p5 for "for('p3')" when I input add_set("u1", ["p1","p2","p3"]); add_set("u2", ["p3", "p4", "p5"])?
I think it is some kind of bug in algorithm, because I think it is equivalent p1, p2, p4, and p5. 

Am I wrong?
