IP-minimization identifies a smaller set of tags that better matches the gold tags; this emerges because other determiners and prepositions evoke similar, but not identical, supertags, and the grammar minimization pushes (but does not force) them to rely on the same supertags wherever possible.
the proportions are incorrect; for example, the tag assigned most frequently to in is ((S\NP)\(S\NP))/NP though (NP\NP)/NP is more frequent in the test set.