snowdown shop

I really like the idea of the snowdown shop and i am really happy with snowbunny nidalee, there is just one thing. I think i read you would get sales of champions you played often. However i got a deep terror thresh while i dont own thresh and only played five games with him ever(only won one). I played a lot of riven, hoping i would get a riven skin (she is my highest mastery now) but i didnt find one in my snowdown shop. I would really ike to know how the system pickes wich skins you will get. Better luck next time.