A different arithmetic of effort
Over twelve rounds, a fighter who spends energy at the rate that would win a shorter fight often has nothing left for the second half, so pacing itself becomes a strategic choice rather than a simple expression of fitness. Trainers plan output across three-round segments, often accepting a slower opening in exchange for a stronger finish, because the championship distance punishes anyone who treats every round as equally urgent from the very first bell.
Cardio as a chosen tactic
How a fighter builds their cardio in training is itself a tactical decision tied to their intended game plan, since a fighter planning to press for twelve rounds needs a different conditioning base than one planning to counter and conserve. This is one reason camps look so different between fighters of similar ability: the training is built around a specific pacing plan for a specific opponent, not around a generic notion of fitness.
Why late rounds carry extra weight
In practice, close fights are often decided by who is still functioning well in rounds ten to twelve, since fatigue-driven mistakes cluster there and judges tend to remember a strong finish vividly. This creates an incentive to hold something back even at the cost of losing early rounds on the cards, betting that a visibly dominant finish will outweigh a modest points deficit in the eyes of at least some scorers.