Are you so sure about this? Have you ever had a situation where you "almost destroyed the second AI city" and suddenly an army of 10-15 units of the strongest units at the moment emerges from the fog? This happens to me regularly. I destroy the AI army, I am already sure that's it, since there have been no reinforcements for several turns, on the next turn I run away, losing everything that was earned by honest work on the battlefield.
If the AI can "conjure" x10 modifier to culture, then why can't it gain 10K coins in a couple of turns?