It seems to me that rank on the ladders is determined almost exclusively by your experience rating. The working theory is that experience is gained/lost based on who you play and whether you win or lose. If you are winning against lower experienced players then you will gain very little experience yourself. Conversely, if you lose a game with lower experience rated players you end up taking a bigger hit to your experience rating. AIs (and possibly those substitued for a dropped player after a match begins) do not yield very much experience either, so if a lot of your ranked games involved AIs that may be one of the reasons your ranking is lower.
As someone else pointed out, Pantheon matches are out of your control. It should be matching you with others around your same rating, so you should be slowly gaining experience if you're completing full matches and winning.
-dolynick
