Again, we must draw a connection between this work and that of Quirk, Brockett, and Dolan (2004) (discussed in Section 3.3) because both treat paraphrasing as monolingual translation.
as outlined in the discussion of that work, Quirk, Brockett, and Dolan use a relatively simplistic translation model and decoder which leads to paraphrases with little or no lexical variety.