Splash art sets

Was going through the splash art of random champions. There is a button at the bottom right hand corner that show all skins in the set that I am currently viewing. For example, if I am looking at Cottontail Teemo, pressing the button would show that Cottontail Fizz is also in the set. However, the button is not available for the Freljord skins. Not sure why it would not be considered a set. Champions that has a Freljord skin include Ashe, Rammus, Taliyah, etc.