Does rerolling for random skin permanent really work this way???

From opening loot boxes I had skin shards for Vandal Jax and Vandal Gragas, which I didn't want, and Heartseeker Vayne, which I already own. I used all 3 of them to reroll a random permanent skin and it gave me Vandal Jax ... So basically I threw two skin shards in the garbage and got a skin that I was using to reroll. I don't think it should work that you can get one of the skins used in the crafting of the reroll, that completely defeats the purpose. I mean, I know it's not a huge deal and it's free loot from Riot anyway, which is great, but I just feel like I just completely wasted two skin shards. Thoughts?