IIRC iready is a formative assessment tool, i.e. the purpose is to diagnose where each student is at, so the teacher knows what to teach next.

There are also summative tests that are meant to measure performance in aggregate, but aren't used for student grades.

This seems unfairly downvoted as a sincere question.

The answer is that the types of standardized tests that these country-wide statistics are based on are not, in the US at least, the tests that generate your grades or your university admission.