I think you missed the forest for the trees. I am sure the student population has some dropoff during summer months but the point is that for businesses that a growing month over month which most of these have since creation, you take the highest number (latest) and annualize it.
I am also willing to bet that the student dropoff is not pronounced. I am more thinking of a business that sells beach umbrellas, they make a lot of sales in the summer months and then next to nothing in the winter months. That would be dishonest.