A team scoring freely from corners and free kicks appears to be in excellent attacking form. Set-piece output is unusually streaky, which makes short-run form readings from it unreliable.
Set-piece scoring is rare and lumpy
Dead-ball situations occur many times per match but convert at a low rate, so goals from them arrive in clusters separated by long quiet periods.
A team can score from several corners in a month and then none for two months without anything about its routines or personnel having changed.
Because a form table samples only a handful of matches, it is very likely to catch a team in one of those phases and report it as a trend.
The underlying process is more stable than the output
What is genuinely repeatable is how many good set-piece situations a team generates and the quality of the chances it creates from them.
Those inputs depend on winning corners, delivery quality and the height and movement of the players attacking the ball, all of which persist week to week.
The conversion of those chances into goals does not persist nearly as well, which is the gap between a team being good at set pieces and a team currently scoring from them.
Specialists concentrate the effect
Set-piece delivery often depends on one or two players, and the loss of a specific taker can remove a large share of a team's dead-ball threat immediately.
Dedicated set-piece coaches have become standard, and a new appointment can change routines enough that the previous season's numbers stop describing the team.
Both cases produce genuine changes, which is why separating set-piece output from open play is the first step in reading whether a change is real.
Defensive form has the same problem in reverse
A defense conceding several set-piece goals in a short run looks fragile, but the same clustering applies and the sample is equally small.
Marking scheme, height and goalkeeper positioning are stable properties, while whether a header six yards out goes in is largely not.
A preview citing recent set-piece goals conceded as evidence of a defensive problem is usually describing variance with the vocabulary of decline.
Separating the two streams
The practical approach is to read open-play and set-piece performance as two different measures, because they have different stability and different causes.
A team whose open-play numbers are steady while its set-piece goals have dried up has not lost form, and the reverse case is not evidence of an attacking revival.