The qualitative analysis of the sentiment lexicons induced by different methods shows that they differ only on those borderline, ambiguous words (such as "soft") residing in the noisy overlapping region between two clusters in the vector space (see Section 3).
sentProp is based on label propagation over the lexical graph of words, so it could be easily misled by noisy borderline words when sentiment clusters have considerable overlap with each other, kind of "over-fitting" (Bishop, 2006).