Forwards brought on late tend to score at a higher rate per minute played than the forwards they replace. The effect is real and has several overlapping causes, none of them mysterious.
They arrive against tired defenders
A centre-back an hour into a match has covered a great deal of ground, much of it sprinting backwards, turning, and jumping from a standing start.
Reaction time and the ability to change direction both degrade with accumulated fatigue, and both are precisely what defending a fresh forward demands.
The substitute arrives with none of that history in his legs, which creates a physical mismatch that has nothing to do with the relative quality of the two players.
The scoreline is usually pushing the game open
Substitute attackers come on when a team needs a goal, and by then the match has usually stretched.
Both defences are further from their midfields, transitions are longer, and the number of chances per minute rises for both sides.
A player introduced into that phase is playing a more chaotic and higher-scoring game than the one that took place in the first half hour.
The instruction is narrower
A starting forward has defensive duties, pressing triggers and a role in build-up across the whole match.
A late substitute is frequently told one thing: get in the box, or run in behind. The narrowness concentrates their activity in the scoring areas.
The same player asked to do the full job for ninety minutes would spend far less of it in the positions where goals happen.
Why the rate is not proof of quality
Per-minute rates from short appearances rest on very few minutes, so a single goal moves the figure enormously.
A player with a handful of substitute goals across a season may have played the equivalent of two or three full matches in total.
Treating that rate as a projection of what he would do starting ignores that the conditions producing it would disappear.
What clubs do with this
Squads increasingly carry a forward whose role is explicitly the late one, selected for pace and penalty-box movement rather than for link play or pressing endurance.
The larger substitution allowance made this viable, because a manager can now change an attack without sacrificing a tactical change elsewhere.
It has turned the closing half hour into a phase teams prepare for separately rather than an extension of the match that preceded it.