Temple = 4 Culture per Turn???

[Desprez]

I'm playing the Zulu and I'm still in the Ancient period, I don't have any wonders built yet, and suddenly I notice something odd.
In all the cities with temples, one of the temples is producing 4 culture per turn, and not the usual 2. (Note that this is not my capitol city, it is not Palace + Temple, it's the temple alone.)

The only other thing I noticed was that all the citizens are happy, (no content, unhappy, or specialists.) But I don't think the city was celebrating this turn. Would that have any effect on culture?

Why is this one temple producting 4 culture?

[Kloreep]

It's probably existed for 1000+ years. All cultural buildings that have existed for over 1000 years or more produce double culture.