It’s unfortunate so many pilots in this industry conflate fatigue with being tired, or a long day, or just a crappy pairing.

Fatigue is cumulative, and everyone’s level varies. Going into a pairing like the one described, most pilots would experience an increase in fatigue, but not to a dangerous level, or even a “call out fatigued level”. Another pilot could already have a higher level of fatigue starting the pairing and it gets worse from there. This is where informational reports are valuable, to track pairings that have potential to push fatigued pilots to more critical levels.

Every pilot is different all the time, and we don’t work/live in a vacuum. Sleep habits, stresses, home life etc etc all play a role in ongoing fatigue accumulation.

Ironically, the high degree of schedule flexibility and manipulation that we enjoy is also the reason that fatigue planning/scheduling is so difficult. Waiving days off, swapping for commutable flights and long days, picking up extra/premium flying in lieu of rest. That doesn’t mean that fatiguing pairings do not exist, but this is why fatigue calls are investigated, to determine what other circumstances are involved.