Remember that aft CG increases fuel efficiency but reduces stability. The increased max altitude reflects that.
Your post seems to indicate that you manually insert a cruise CG. I have never seen that practice, and do not know why you would do so. If so, why do you do it and how do you calculate it?
the .82-.85 range tends to place you in the center of the available cruise speed window at high altitude, between the yellow and red arcs, thereby giving you the best compromise between Mach buffet and stall buffet limits. If you search through the old versions of Boeing AERO magazine, you will find the discussion of turbulence speeds, and the recommendation to err on the high side of the limit if at all.