? about population and gold

[Cor2]

In one game where i was playing growth 3 i noticed that while the population was rising, gold was dropping in my capitol. here is a note i took:

pop=36730 gold= 572

a few turns later : pop= 38530 gold = 569

there where no events,no dominion changes,no unrest and no tax changes.

Any idea on what could cause that?

thanks

cor