Your observations sound familiar to those in some areas of the United States.

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There is an extreme need to pretend that some differences do not exist, even in achievement.
It is ironic because in sports/athletics it is widely accepted as natural that some will excel; These individuals are celebrated. Meanwhile in academic/intellectual endeavors uniformity is mandated; Those who excel may be cut down (tall poppies).

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teachers are supposed to grade objectively against grade level standards and not on a curve
In some schools, gifted students may be graded on much tougher, more time-consuming assignments as a means of "differentiated task demands", resulting in deceptive, uneven grading.

In some high performing areas teachers may be credited with the performance of pupils even though there may be a strong program for uniform outcomes with little or no gifted support; Parents may be sacrificing greatly to provide weekend, summer, or afterschool enrichment for their children so they may learn something new. Based upon high student performance, the teachers to whom these students are assigned are then deemed superior and draw higher salaries although the students' high performance was based on parental decisions, sacrifice, and actions in support of the student.

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dirty little open secrets substantiated by test results nobody wants to hear
Yes, this may be similar to the observation that children in practice programs may have high GPA... but with little outside evidence to corroborate high academic achievement... thereby hinting at inflated grades for some students (based on uneven grading practices).