In addition to normal random variation, there are some other issues which may play a major role in sim performance:
- Salary cap - the higher the cap of the league, the more variation you'll see between real life stats and sim stats.
- Ballpark - ballpark can have a huge impact on simstats - not just your home park, but your opponent's parks as well
- Opponent lineups - if you have a strong left handed starting rotation but you ended up in a division with great right-handed hitters, this would negatively impact simstats.
- In game fatigue - it's possible that your pitcher is tiring early in the game but not being pulled until it's too late. For example, if you have a pitcher that threw 300 IP but had 45 starts, that would roughly equate to 100 pitches per outing (300 x 15 pitches per inning = 4,500 total pitches/45 starts = 100 pitches per outing). If the pitcher has a max pc setting of 120, he's being allowed to pitch deeper into games and may be pitching while fatigued which could lead to big innings.