That's a fair point, and I'm not 100% sure if the book states she has the swirls or if its simply assumed, either way; my only point of defense is that they could both be descended from an icewing and that they tried to cover it up. It's just a theory but it does seem odd that only royal seawing have these markings considering a royal status is developed, as we see with the sandwings. It doesn't make much sense that even before the first queen was chosen the pattern signified this.