Are cluster names from Zingg trainMatch() always sequential or can some numbers be skipped?

Hi! I noticed that when Zingg outputs the clusters from the trainMatch() method, the cluster naming is not sequential. For example there is cluster 3 , Cluster 4 but cluster 5 is missing and it jumps to Cluster 6. The number of total records is still the same though. Can anyone explain if this is an unexpected result or it is not necessary that the clusters are named sequentially?