A possible reason that the random synchronous binarization method can outperform the baseline method lies in that compared with binarizing SCFG in a fixed way, the random synchronous binarization tends to give a more even distribution of rules among buckets, which alleviates the problem of edge competition.
since the high-frequency source sub-sequences still have high probabilities to be generated in the binarization and lead to the excess competing edges, it just achieves a very small improvement.