They can fluctuate quite a bit or at least they could in the past. I had one year in M07 where Favre ended up going off for 70 TD's (all simulated) to win the MVP but that wasn't the worst part not only was that number ridiculous the next 5 QB's in TD totals were all over 50. They year after that Favre went for 20 while the league leader had 30.
Now I don't expect to see swings like this ever again as 70TDs is just stupid but there is a chance that it was must a down year. Sim a few more years and see if the trend continues then it is clear that there is a problem.
To the guy that said no one had a passer rating over 100, what was the highest passer rating in the league? Having a year over 100 passer rating is a great year and it wasn't that long ago that a 85 passer rating was considered a pretty good year (look at some of the career passer ratings of players who played in the late 80's and early 90's). I understand the game has become a passing game but if the top 3-4 guys have an upper 90 passer rating then I see no problem with that.
EDIT: I just looked at the passing statistics for last year and only 4 QB's had a passer rating over 100 (Brady, Rivers, Rodgers, and Vick) and only one (Brady) was over 102. Rivers came in at 101.8 which could have just as easily been under 100 with one more interception, less yards, etc.