Free Champ/Skin bug?

I have exactly 10 champions left, but received a skin. I ain't mad that I finally got something for free, but I was under the impression I would get a free champ w/ that number left. Which is why I wasted 3150 on MF to prevent myself from getting her.

Any way this can be fixed or did I misunderstand the meaning of "at least 10 champions left"?

Edit - for clarity, I know about the 520 RP thing. I have two/three 4800 champs left to purchase - Caitlyn Talon and Graves. The rest are 6300s, which are Darius Lucian Lissandra Zyra Lulu Zac and Varus.