Arena brackets, how do they work?

I had been doing a good job of maintain8ng top 20 in arena. I'm level 77, almost 78, and have an arena power around 32,000.

I'm out of the top 20 now, and there are a number of people between me and the number 20 slot at level 81-82 and 42,000+ arena power.

I can accept dropping, but I'm curious to know where these people came from. Were they always here, and just now got around to spending Christmas cash, or what?