Clean sheets are the standard shorthand for a well-organised defence. They measure a combination of organisation, goalkeeping and opposition finishing, and the last of those is almost entirely outside a team's control.
The measure is binary and therefore coarse
A team that conceded one poor goal while restricting the opponent to nothing else is recorded identically to one that was overrun and lost heavily.
Conversely, a side that survived a barrage through one exceptional save is credited with the same achievement as one that never allowed a shot.
Compressing a match into a yes-or-no outcome discards most of what happened defensively. The number of shots faced, their location and the pressure applied to each of them all vanish into a single tick or cross.
Opposition finishing swings hard
Conversion rates vary enormously from match to match, so identical defensive performances produce different clean-sheet records purely through what the other team did with its chances.
Across a season those swings partly average out, but a run of ten matches is far too short for that to happen.
A clean-sheet count over a short period is therefore describing the opposition's finishing as much as the team's defending.
Goalkeeping is bundled in
Shot-stopping is a distinct skill from defensive organisation, and a goalkeeper in good form can carry a poorly structured defence for weeks.
Separating the two matters because they change independently. A goalkeeper's form fluctuates while the defensive structure stays constant, and the reverse also happens.
Measures that assess the quality of shots faced against the goals conceded make that separation possible.
What actually describes defensive organisation
The quality of chances conceded is the closest available measure, since it reflects where opponents were allowed to shoot from and under what pressure.
Where a team allows possession, how quickly it recovers the ball after losing it and how often it concedes counter-attacks all describe the structure directly.
None of these depend on whether the opposing striker happened to finish well on the day, which is precisely what makes them steadier across a short run of fixtures.
Why the shorthand survives
Clean sheets are easy to count, appear in every match report and connect neatly to a result, which is a lot of practical advantages.
They also carry genuine information across a full season, where the noise has had time to settle and the count reflects something real.
The error is using a season-length measure on a ten-match sample, which is exactly what a form guide does.