I know that they scale with turns, but not with difficulty. It seems that an AI that lands on turn 20 will get the exact same bonuses when landing on <whatever the lowest difficulty is> and on Apollo. That does of course lead to balance problems, you can't make the bonuses large enough to make up for the general AI stupidity when confronted with situations where they're able to build/do tons of important stuff at the same time and still have them be weak enough to not pose an unwanted difficulty spike for low difficulties.
But yeah, main problem is probably the AI that just doing weird things when it has a ton of stuff to choose from with very delayed starts. After all, it uses a simple priority system that is built around the idea that there aren't a ton of things available - but when the AI gets enough tech points after landing that it has 6 buildings available that have a higher total priority than settlers, then it will just build them first, without even thinking about expanding. At least that's my theory to explain my observations that AI is slower at expanding the later it lands. Not saying this is a fact or something that always occurs though, I only paid attention to that a few times - but every time I did, it was the same thing. The ones who landed earlier got rather wide empires and became relatively strong (if not conquered by another Civ that lands late right next to them), the ones who landed late stagnated if they didn't find anyone to conquer.
