It appears to me that the ratings are in fact a real time calculation that we just do not get to see except at the end.
A player who subbed in for 15 mins and got an 8 was playing at an 8 level during that 15 mins. Goals and assists do have a big impact on boosting ratings as yellow/red cards do the other way. There are clearly other factors involved in the ratings though as I have recently seen my best player by far get an 8 and MoM as a ST without a goal or assist while the players that scored for me obviously didn't win MoM even though they also had 8 ratings.
It suggested to me that his overall level of play was very high. Controlling the ball, passing, creating chances, getting shots on target, and pressing/winning the ball all likely were very good for him and made him the best player in the game even though the actual scoring plays were by others.
The ratings for defenders also suggest this. I regularly get high ratings from CB's and FB's with especially CB's winning MoM quite often without goals or assists, even the occasional MoM to the keeper happens, though rarely for me as my opps do not get many chances usually leading to only 1-2 saves and sometimes not even a single on target shot. I think they have made the GK ratings better because they are no longer being punished for matches where they didn't have much to do where last season I was seeing 5 ratings for matches that my GK never faced a shot in, which is just wrong IMO (that should always be a 7 to me). It could just be that they made the ratings for GK/CM higher without real change, hard to be certain.
I would like to see them generally fix the way the engine treats CM's. No matter how I play they get very few assists. I regularly will see 9 goals and 0 assists over a season or something similar. If a CM has enough chances to score 9 goals he should have many more assist opportunities IMO. My way of evening things purely to make my record book look less weird is give the CM the CK specialty and duties. It is artificial for sure but at least I do not have to look at the records and see 9/0. 9/21 looks much nicer. It also used to be a way to help raise the CM's overall match ratings but now that isn't really needed.
I wonder what my CM from the last team would look like now........He was getting 20+ goals and 15+ assists a season, competing with the AM's in the score chart. He would have to be getting a lot of 9/10 ratings now.