From the error analysis, we can conclude that the added words do not correspond to the words that are needed in the test domain, which means that the HCRC map task corpus data are not similar enough to the CReST data.
we can also conclude that even if there were a larger overlap, there is a high chance that those words would be mistagged by the ensemble of taggers so that adding the new words would result in a deterioration of the performance.