Quote from AMT4SWA:
That is a byproduct of price trading back up to levels from which a previous contract was the current month contract (as we traded down through the lower 900's to our 665.75 low of year run). This deals with the more complex multi-contract tracking, and the fact the market made very dramatic price range moves in a few month period of time.
BTW, I was a seller in most of those rallies into previous zones (from previous contracts) and the divergence was as aspect I took into account.