I can try to explain it another way:
The school are reporting their SEN data as low to you because:
School SEN group = -8
National SEN data = -6
However, they are aware that there were 2 students out of 12 who performed significantly below the national average and are not concerned that this is an ongoing issue- it’s just an outlier. / OR these below-average scores were across the board, they are concerned, and their school improvement plan will focus on improving outcomes for this group.
Other groups in the school:
Students with orange socks:
School orange sock group = -5
National orange sock group = -8
P8 is negative, but the school are quite happy, because they are doing better for these students than other schools are.
Students with yellow socks:
School yellow sock group = +1
National yellow sock group = 0
P8 isn’t phenomenal, but school are quite happy.
School know that 30% of their intake have orange socks. They report their concerns to parents about the SEN group, because these students have performed worse than the national average of students in this group. They don’t report the orange sock group, because they are actually quite pleased with this result: it’s better than the national average for this group, even though it’s below national average of students as a whole.
Overall, their data reveals negative P8 for their students.
However their contextualised data is actually only a concern in one area- SEN. They have told you about this.