Another possible reason for the accuracy is the constraints placed by unification-based grammars.
given a set W of words and a set F of feature structures, an HPSg is formulated as a tuple, g = L, R , where  Previous studies (Abney, 1997;Johnson et al., 1999;Riezler et al., 2000;Malouf and van Noord, 2004;Kaplan et al., 2004;) defined a probabilistic model of unification-based grammars including HPSg as a log-linear model or maximum entropy model (Berger et al., 1996).