Some units, like the Celestial Carp, will eat or otherwise dispatch a certain amount of population each turn, substracted from the province they are in. Hinnom is a nation notorious for having a lot of hungry giants.

While generally detrimental to your economy, it can be entirely negated by putting the unit in a 0 population province. It can also generate some amount of civilian corpses (10% of the population killed) each turn, which might be of use to Death magic users.