Looking at the parser's output, the reason for this poor performance is that the adjacency condition in the distance measure is approximating subcategorization information.
in phrases such as PPs and SBARs (and, to a lesser extent, in VPs) that almost always take exactly one complement to the right of their head, the adjacency feature encodes this monovalency through parameters P(STOP|PP/SBAR, adjacent) = 0 and P(STOP|PP/SBAR, not adjacent) = 1.