You might have noticed this because the AI seems to be stronger in the earlier part of the game. After railroads, their combat tactics don't come close to a human players. Combine their early strength with the fact that so many of the civs have early age UUs which augment their strongest part of the game with a golden age, and they can seem quite a bit more powerful early on than they do later in the game. But the same thing can be said about a human player too. Someone playing persia will have far more success in general than someone playing germany in an ancient war. But if you advance the time frame to tanks, all things being equal, German panzers will have far more success in a war than persian tanks.