Oh there must have been a mistake on the voice over. If i measured 10”. Then that’s what it is. But also 15 works too( it just means your sleeve will be puffier. Which is even better.
Based on the actual top design. The cups have a rounded look with very Minimal gathering around it( makes it less puffy) Hence, the circle( in a cup shape). But if your style preferences want a puffier /fuller cups, that’s another way to do it.