We put a little man inside the game, feed him nothing but breadcrums and waterdrops. Spin him around 7 times on a big rollercoaster ride and then he just comes up with just numbers, all the time.

To be slightly more honest, it seems that massing tons of units seem to increase it, but I have no idea of the specifics.

Sincerely
~ Kongdej

Sometimes power rating can go very high because of resources, if you play higher difficulty levels AI can have tremendous amount of money or prestige, which pumps up their power rating.