There a lot of variables at play here: 

First thing I can think of is that their cultures might have influenced which Era stars they went for in Era VI - i.e. the reason why we might see a drop-off in science Era stars is because they chose cultures that gave them Era stars that gives you more fame for other tasks. 

Second variable I can think of is the distinction between triggering the end-condition vs. the actual winning condition which is Fame. 

According to the graphs you catch up to Black's Fame lvl around turn 400. Before turn 400 the Black player is not even close to having the research to trigger the end-conditions. And if they trigger the end-condition after turn 400 they would lose- since you caught up in Fame already. In other words, ending the game would have been a losing move for the AI after turn 400+. 

So the AI had to win the game between turn 300-400 - but as we see on the graph over research and science they are not even close at pulling something like that off at this time. And remember, what is important for the AI to win is the Fame lead - so widening that gap is more important than doing research (for the AI)- which can probably explain why their Research Era Stars fall over time (if indeed they had cultures that explain this). 

Cool analysis though :)