That’s because the tests are way too easy to make meaningful distinctions at the top end. For instance, the math portion of the SAT is very low level, and yet a single silly mistake could knock an 800 to a 770 or 780. Are we testing rote conscientiousness, or true ability? We need different tests, or else we consign ourselves into an ever more crowded madhouse in which the top 5% can make themselves look like the top 1%.