Extra time is not simply thirty additional minutes of the same match. The goals scored in it arrive from different situations and at different points, because both teams are managing an entirely new set of constraints.
Fatigue changes what is physically available
Players entering extra time have already covered a full match, often at high intensity, and their capacity to sprint repeatedly has dropped substantially by that point.
Distances between players stretch because pressing arrives a step late, which opens space in midfield that simply did not exist an hour earlier in the same match.
The chances created reflect that change: more shots struck from distance, more counter-attacks run into open pitch, and far fewer goals produced by sustained pressure.
Set pieces become disproportionately productive
Marking a corner requires concentration and the strength to hold a position against contact, both of which are diminished after two hours.
Defenders lose runners they had tracked comfortably earlier, and second balls are contested less aggressively.
Teams that recognise this deliberately seek corners and free kicks in extra time rather than trying to play through a defence.
The shootout distorts risk calculations
A team that believes it will win a shootout has an incentive to avoid conceding rather than to chase a winner.
That belief may come from having a strong goalkeeper, from confidence in its takers, or simply from having practised more thoroughly.
Where both teams reach that conclusion, the final ten minutes become notably passive, which is the opposite of how the period is usually portrayed.
Substitutions arrive with a specific purpose
The additional substitution permitted in extra time is frequently used for a fresh runner or, near the end, for a specialist penalty taker or goalkeeper.
A fresh player against exhausted opponents produces a mismatch larger than any that existed during normal time.
Single goals in extra time are often traceable directly to that mismatch rather than to a tactical change.
Why the timing clusters at the ends
Goals concentrate in the opening minutes of each half, when a team introduces a change during the short break and the opposition has not yet adjusted to it.
They also cluster in the closing minutes of the second period, where a side unwilling to face a shootout abandons caution and commits everything forward.
The middle stretches are unusually quiet, which is why extra time so often feels slow until it suddenly does not.