I understand the AI's teching problems on water maps, but everyone is limited in production. How does that hurt the AI more than the human?
The AI gets flat, percent bonuses to tech, production, etc. These bonuses aren't malleable. They are the same in every city, for every point of production, beaker, and so on. In short, the AI grows by getting more stuff. It can't grow by directing available resources to narrower ends. That's what the Human does.
So basically, maps that give the AI a larger number of cities and more stuff will lead to harder AI. Meanwhile, a human player is much better at realizing, hey, there's space for 3 cities on this Island. My capital is the only one that can work these double Stone tiles. These others have mainly Fish nearby, and I'll have to settle for priority buildings there because others will come too late. And I need to rely on production in my Captial for the invasion force I need.
For example, Human will build the NC early, then beeline some tech, then build what the tech enables and execute mission. Little resources are wasted on anything not having to do with that mission. So the human is able to put the rewards from that mission to use much earlier. By contrast, the AI will expand randomly, then decide to build the NC after the hammer costs have escalated and the Empire's Science is coming from other sources. Then it will wander into Chemistry, for example, 20% later than it should've. The result is that the benefit of an extra hammer per Mines via Chemistry is cut. Also, the AI might build 20 units and then only effectively engage with 10, and the hammers spent on the extra units go to waste. A human would have built 10 units, then built something like Workshops, then returned to building units only in the event that the first 10 didn't do their job.