The references to the top 50% all refer to the median. As you probably know, the median is the number in a set of numbers that falls exactly in the middle after you have ordered the set of numbers from lowest to highest.

So, in other words, a 3.2 GPA curve means that 50% of the class will get above a 3.2 and the other 50% of the class will have something below a 3.2.

As a final note, St. John's scholarship offer means that if you are above their median GPA, you will keep the scholarship, but if you aren't, you lose it.