How do you get a skin loot?

I have a question about how do you get the skin loot from the random chest you opened. For the last 8 chests I opened, I only got about 6 champions loots and 2 skin wards. There wasn't a single skin loot for me. even though I owned all the champions, there still give me all the champion loots. So what the chances are to get a skin loot? Seems like some of my friends are only getting skin loots instead of the champions. That's very weird I think.