If the tests are about drills and skills not the rigorous concepts, then good test results will make the students and teachers think they know the concepts, when they don’t.
Being able to recite the addition table is not the same as understanding addition as a recursive function. But memorizing the addition table makes people think they have nothing more to learn to know addition. They are wrong, but the good tests reinforce their misplaced confidence.
K-6 has this built in to its entire approach. It is worksheet driven. The worksheets themselves are not based on understanding Peano Axioms or the concepts of elementary math that build on them. Thus the worksheets are pieces of math, but have huge gaps in the structure of math.
However, the students and teachers don’t know this. They think they understand the structure of math when they don’t. The result is when they try to build on it, they struggle. And the struggle is worse that they don’t have access to an explanation of what was wrong in their understanding of the foundations they are trying to rise from.