Two strikers with similar ability can end a season several goals apart because one plays international soccer and the other does not. The mechanism is availability, not quality.
Missed club matches are the direct cost
Tournaments and qualifying windows remove players from club schedules, and depending on the calendar a forward can be absent for a run of league fixtures.
Those are matches in which he could not have scored, and season totals record the absence as a shortfall against players who were available throughout.
Rankings that use goals per season rather than goals per ninety minutes therefore penalize international forwards before any question of form arises.
Travel is the larger and less visible cost
An American forward called up for a match in another continent may cross many time zones twice inside a week and play a competitive game in the middle.
He then returns to a club fixture within days, and the physical state he brings to it is well below his normal condition.
Coaches often rest such players, so the effect appears as reduced minutes for weeks rather than as a single poor performance.
Injury exposure scales with matches played
More competitive matches means more opportunities for a muscle injury, and the periods immediately after international travel are recognized as elevated risk.
A forward who loses a month to a hamstring problem after a call-up has effectively had his season shortened by the national team commitment.
None of that appears in a comparison of final goal tallies, which treats the two players as having had the same opportunity to score.
Major tournament years compound everything
A summer tournament removes the off-season, so a forward begins the following campaign without the recovery block his peers received.
The pattern is familiar enough that clubs plan for reduced early-season output from returning internationals as a matter of routine.
With a World Cup hosted across North America, a large group of players will face exactly this compression, and their next club season should be read accordingly.
Rate statistics fix most of the distortion
Goals per ninety minutes removes the availability problem cleanly, and it is the first adjustment any serious comparison of strikers should make.
It does not capture the fatigue effect, since minutes played while depleted count the same as minutes played fresh.
Which leaves a residual advantage for forwards who are not selected internationally, and it is worth naming rather than pretending the comparison is clean.