early in the season, damping effects are used so your players don't seem unnecessarily tired.
remember that fatigue is prorated. so after game 1, the sim assumes your game 1 starter will pitch 7 innings x 162 games. and if you have a 6-PA game for your hitters it will assume that all your hitters will have 6PAx162 games. that would make them excessively tired, so damping is applied.
it's still possible to push someone so hard that you fatigue them through the damping effects. if you had a 100-PA hitter and played him 20 PA in the first 6 games, he'd still be below 100%. this prevents people from using guys like Ted Williams for their whole first 15 games.