URF Champion rates are so off

I feel as if the rates to get certain champs are higher than others. I played SNowday ARURF on both the PBE and main client versions and there's such a "weird" cycle of seeing the same champion every 1 or 2 games (often times in a row) and some are rarer (ex. Shaco, Draven, Syndra). Anybody else notice this?