<p>Self selection comes in that for a subject such as Physics, the majority of takers WILL be amongst the best in their fields - so if only the top 10% of Physics students take the Physics SAT then the 50th percentile is actually the 95th percentile nationally.</p>
<p>Consider maths 1 and 2 - because only the top maths students take maths 2 (can’t find numbers, but assume that 90% of people take 1 and the top 10% take 2). Then the 90th percentile in 2 is the 99th nationally (for maths) and the 99% percentile for maths 1 is only the 89th nationally (Of course practically this is not true because there is overlap - some of the top 10% of maths students take maths 1)</p>
<p>This is what wikipedia has to say:</p>
<p>The differences in what scores mean with regard to percentiles are because of the content of the exam and the caliber of students choosing to take each exam. Subject Tests are subject to intensive study (often in the form of an AP, which is relatively more difficult), and only those who know they will perform well tend to take these tests, creating a skewed distribution of scores.</p>
<p>Certain test are more skewed</p>