Spain's domestic cup is structured to put its biggest clubs on the road against small opponents in single matches. The design is deliberate, and it produces the results it was built to produce.
The format inverts home advantage
In the earlier rounds, the tie is hosted by the club from the lower division, regardless of which side would normally be the more attractive host.
Ties are played over a single match rather than two legs, so there is no second fixture in which the stronger club can correct a poor performance.
The combination removes the two mechanisms that usually protect favourites in knockout football, and it removes them at exactly the stage the gap in quality is widest.
Small grounds are a genuine variable
Lower-division stadiums have tighter dimensions, different pitch quality and crowds positioned close to the touchline, all of which compress the space technical teams rely on.
A side accustomed to a wide, immaculate surface has less room to play through a compact defensive block, and the block itself has less ground to cover.
Playing conditions therefore narrow the difference in ability rather than merely providing atmosphere.
The calendar makes rotation compulsory
Cup rounds are scheduled in midweek slots surrounded by league and European fixtures, so leading clubs field changed sides out of necessity.
A rotated eleven is not simply weaker. It has played together less, which affects timing and coordination more than individual quality does.
Lower-division opponents, by contrast, treat the tie as the match of their season and prepare for it with everything available.
Why small clubs accept giving up a home tie
Hosting a major club generates income that can rival a substantial part of a small club's annual revenue, between attendance, broadcasting and commercial activity.
Clubs occasionally move the fixture to a larger nearby stadium to increase capacity, trading some of the home advantage for a bigger gate.
The competition is designed with that transfer of money in mind, and the sporting jeopardy is partly a by-product of the financial arrangement.
What it means for previewing a cup round
League positions are a weaker guide here than in almost any other fixture, because the format is actively working against the stronger side.
The useful questions concern rotation, travel and the surface rather than the gap in the table.
Upsets in this competition are not anomalies to be explained away. They are the intended output of the rules.