The closer the contract comes to expiration, the closer it is to the cash index (parity). When the new contract is instituted, it reflects the market participants expectations of it's future value. That is plain vanilla (no sprinkles).
I just trade YM, did not notice the difference between sept.Ym and June YM.
just one more question, when june YM epire, if I hold YM (bought it at 8250), I do not sell it, what will happen, just convert it into cash? or i have to sell it?