Have you pulled with that set of channels before? If you did, then the replication will pull the documents in those channels that have changed since the last time.
In other words, a plain replicator (no channels specified) has a separate state from a replicator with one set of channels, which has a separate state from a replicator with another set of channels.
If you call
reset on a Replicator instance, it will clear its state and fetch everything. (But this will be slower.)