@OHToCollege, I would say that is sloppy writing on the part of Vandy. It’s easier to explain the 50th percentile by saying 50% scored 32-34 than saying something like 64.3%. If ACT scores were more differentiated (more like the SAT), it would be easier to divide up the population into neater quartiles. However, even SAT percentages can be misleading: if you look at SAT scores at Vandy, you see that the middle 50% for math for Class of 2019 is 720-800. Surely, you wouldn’t claim that 25% of Vandy students have a math score above 800.
Also, please note that the Vandy explanation is not meant to be exact: contrary to the example given, not all 1600 Vandy students even submitted ACT scores.