For less frequent POS classes (those that dominate the macroaveraged measure, especially verbal POS), sequence information and "long-distance" context is probably more stable and can be exploited better than for NN, NNP and JJ.
there is still a dropoff from the baseline for BIO; we attribute this to the larger differences in the transition probabilities for BIO vs WEB (Table 4); the sequence classifier is at a disadvantage for BIO, even on a macroaveraged measure, because the transition probabilities change a lot.