Read the whole curve
Early-set speed and control establish the session. Later-set rep loss shows how fatigue accumulated under the same load and rest.
The goal is not to preserve an arbitrary reserve on every set. It is to keep the work in the target muscle while the sequence becomes genuinely demanding.
Record where the limit arrives
If the target is 12 and a later set ends at nine clean reps, record nine. That is useful evidence, not a failed workout.
When the limit arrives too early, reduce load. When it moves later across the series, the programme is progressing.