The reason for the worse results is that in our experiments, the oracled PPA always identifies more prepositions attached to verbs than the PPA classifier, therefore more prepositions will be given semantic roles by the SRD classifier.
since the performance of the SRD classifier is not high, and the segmentation subsystem does not always produce the same semantic role boundaries as the CoNLL data set, most of these additional prepositions would either be given a wrong semantic role or wrong phrasal extent (or both), thereby causing the overall performance to fall.