They change MMR factors on a pretty regular basis. Something like every week and in some cases day-by-day, they do this because of the very tiny playerbase, the population density in certain MMR and player level ranges. Probably there are other deciding traits involved but these seem to be chief among them.
Specifically, the MMR factors which get tweaked are going to be how much reward a certain percentile should be allowed to give which is influenced by your score at the end. They can change how much score you get from that percentile or how much reward you get from that score or a combination of the two. It constantly evolves to suit MSE's mystical needs.
Just a month ago or so you could get a 10.0 pretty easily as a Raider or Antagonist. Before that 10.0 was a pretty mythical thing but then everyone was getting it all the time. People who just turned on the game for the first time were getting them, I know because I mentored a few of those people. The rewards for 10.0 back then were about as good as they are for a 8.0 now.
The score is pretty meaningless. That being said, if you want a high one then you should be ready to face that appropriate challenge. Which I must say that 60% MMR used to be a death march, so if it still is then that makes sense to me.
