We find after analyzing the data that, query pairs like "_dÈ'Ë p (Justin Bieber height)" and "_dÈ'Ë p N-(Justin Bieber height and weight)" are quite common in users' search sessions, in which a query is expanded by adding a word closely related to the original query words.
many closely related non-paraphrase word pairs, like p (height) and N -(weight), get large a − scores and thereby penalized by the lexical score features.