Simultaneously streaming database replication and pg_dump, yet observing zero lag

We have a streaming replication setup, where the replication slave runs pg_dump every hour. Our lag monitor shows an
expectedsawtooth shape, where pg_dump runs coincide with a climbing lag, which ends abruptly when pg_dump is finished.
Thisis, to the best of my knowledge, expected behaviour. The lag pattern occasionally disappears though, and when
lookingat system processes we see that the database is recovering WAL data, and pg_dump is also running. How is this
evenpossible?
 


FWIW, Master is running PostgreSQL 9.3.5-2.pgdg12.4+1 and the slave 9.3.6-1.pgdg12.4+1, and both running on Ubuntu
12.04LTS.
 

Cheers,
Matt