The ES SEP future is 2.5pts behind the SPX -- for the last 10-15 years, it's been (at the transfer to a new "front month") ~8pts. This is the second time I've noticed this -- MAR-JUN and DEC-MAR being the prior two that I can recall with surety.
We haven't seen any material rho ("interest rate") issues in a long time. I haven't dug up the equations (and don't recall them off the top of my head), but I suspect that the futures are gaining value because they are *suspected* of being in a higher interest rate regime than the current. By how much? Well, I'll start by backing out ~6pts of ES worth out of 2440 or so, and aim that over 3 months. I don't know, but it makes sense to me. (Enough to be my straw man.)