Very normal. 162 games is going to mean something like 720PA, depending on lineup position, how often they get replaced for defense or PH, etc.
WIS gives position players a 10% bonus, so someone with 660 actual PA/162 will still be at 100% if they accumulate 720PA over 162 games. But if at any point their pace is to go over that, they will show fatigue. And obviously anyone with fewer real life PA/162 will fatigue faster. Bottom line - you either have to rest players periodically, or be willing to play them fatigued.
I recently posted a long thread that went into some detail on how fatigue works. I'll bump it for you.