Novelty of stimulus is a huge factor, especially as training continues over years. Failure from a set of 20 is very different than failure from a set of 5, and

Novelty of stimulus is a huge factor, especially as training continues over years. Failure from a set of 20 is very different than failure from a set of 5, and bodybuilders will periodize their training to cycle through the different flavors of stimulus. I think a big contributor might be neuromuscular adaptation. Cycling through those different intensities over training periods measured in months will make this apparent anecdotally.