Because if you cut them 6.5 they'll turn out too small. To make half square triangles you add 7/8ths to the *finished* size which is the measurement after it's sewn into a quilt. So if you're making half square triangles to go with 6.5 inch squares they have to start at 6 7/8ths. Since 7/8ths is hard to cut and half square triangles don't always come out perfect you just add that extra eighth and square them up after you make them. When you square them up you'll trim them to a perfect 6.5 to go with your other 6.5 inch squares.