A striker who scored freely in one competition often does not in another, and the usual explanation is league difficulty. Role change accounts for more of the shift than most people assume.
The chances a forward receives are a team property
A striker does not create most of his own opportunities. They arrive from the way his teammates attack, the width they use and how often they get behind a defense.
Moving clubs replaces that entire supply system, and a forward can join a better team that produces fewer of the chances he is good at.
Which is why the same player can be prolific for a modest side and ordinary for a stronger one without his ability having moved at all.
Set-piece and penalty duty transfer least reliably
A forward who took every penalty at a previous club may find the responsibility already assigned, and that alone can remove several goals from a season.
The same applies to attacking corners, where a striker's aerial goals depend on being the designated target rather than on his heading ability.
Comparing non-penalty, open-play goals before and after a move gives a much cleaner picture of whether anything real has changed.
Positional demands often shift without being announced
A new coach may want the same player pressing higher, dropping deeper or holding the ball, all of which reduce time spent in scoring positions.
Forwards asked to link play take up positions further from goal, and their shot volume falls even while their contribution rises.
The goal column records the change as a decline, which is one of the more common misreadings in public evaluation of strikers.
Adaptation takes longer than a transfer window's patience
Understanding a new set of teammates' habits, learning when a winger will cut back rather than cross, is a matter of months rather than weeks.
Judgments made after a handful of matches are drawn from a sample too small to distinguish a struggling player from a settling one.
Strikers who eventually succeed after a move frequently look poor in the first stretch, which is why patience is the most common ingredient in the successes.
The reverse move is instructive
Forwards who drop to a less demanding competition sometimes score less rather than more, which contradicts the simple league-strength model.
They often arrive at a team that concedes possession and plays with a low block, giving the striker fewer touches in the box despite weaker opposition.
Which supports the conclusion that a forward's output is a joint product of his ability and his team, and that separating the two is the whole task of ranking strikers.