You're a little late to the party but these will catch you up:
http://www.chess.com/forum/view/livechess/new-logic-for-auto-pairing-and-seek-graph
And:
https://support.chess.com/customer/portal/articles/1622811-how-are-seeks-matched-in-live-chess-
Recently, I noticed that the option in live chess to set the min/max rating search parameters is gone for rated games. I prefer to get matched with opponents with a stronger rating than me, but now I have no control over this. What is the explanation for this change?