In a league with promotion and relegation, a club's opponents change from one season to the next. Any ranking that uses past results has to handle that discontinuity.
Results are only meaningful against known opposition
A rating derived from wins and losses is a statement about the teams beaten. When a club changes division, the meaning of its results changes with it.
A promoted club arrives with a record built against weaker opposition, so a naive rating overstates it substantially in its first season.
Relegated clubs present the reverse problem, carrying a poor record accumulated against stronger opposition into a division where they are likely to be dominant.
Rating systems bridge divisions through shared opponents
Elo-style ratings handle this without special treatment, because each club carries a rating that already reflects opponent quality, and cup matches between divisions link the tiers.
Those cross-division fixtures are few, so the calibration between tiers is uncertain and tends to be revised sharply at the start of each season.
Coefficient systems have no equivalent mechanism at all, since they only observe continental matches and a promoted club has no continental record. Such a club enters seeded at the bottom whatever its actual standard.
Closed leagues avoid the problem entirely
Competitions without promotion and relegation keep the same membership indefinitely, so every season's results are directly comparable with the last.
That stability makes historical ratings far more reliable, and it is one reason statistical analysis developed earlier in sports organised that way.
The trade is competitive. A closed league has no mechanism forcing persistently weak clubs to improve, which is why draft systems and salary caps exist alongside it.
Financial cliffs make performance discontinuous
Relegation removes a large share of a club's income at once, forcing player sales that weaken it further, so a single relegation can produce years of decline. Contracts written for one division rarely survive a drop to another.
Parachute payments soften the drop and give relegated clubs a spending advantage in the lower division, which distorts competition there.
Ratings that ignore these financial effects will consistently misjudge recently relegated clubs, in one direction immediately and the other later.
Season length differs between tiers
Lower divisions often contain more clubs and therefore more fixtures, sometimes with additional cup competitions and play-off rounds.
A promoted club has usually played a longer and more congested season than the clubs it joins, arriving with less recovery and a squad built for volume.
Any comparison of form across the boundary therefore compares seasons of different shapes, which is a further reason cross-division rankings carry wide uncertainty.
