I'm silver and randomly got given challenger border and unowned skin for a match.

So yesterday I played a game at a gaming arena. I'm pretty sure they didn't have league unlocked as not all the champs were available. Anyway I played an unranked 5v5 game and picked janna. I have the weather forecast skin for her and chose that. However when the loading screen came on, it said I was in the star guardian janna skin, which I don't even possess. On top of this I had this really fancy boder. I didn't recognise it but looking around on the internet afterwards, it appears to be the challenger border. I'm only silver. I only played one game so not sure if it was just a one off thing or not. Does anyone know why this happened?
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