I got 4 gangplank skins on my snowdown shop. Care to explain?

So I went to that snowball shop thing, and 4/6 of the skins are all gangplank skins. I've played GP some around the end of last season, and haven't played him since. Is it based on highest mastery, because he's only my 8th most played.

Also: i already have the captain GP skin, I don't want 2 skins for 1 champ.