Why did Jasper and Rosalie graduate in different years?

I wondered that too for a bit, after watching Eclipse.

In the books, Jasper and Rosalie were said to be twins and they graduated with Emmett the year before Edward, Alice, and Bella graduated.

In the movies, Jasper and Rosalie were said to be siblings, but they weren't twins. Jasper was said to be Rosalie's younger brother.