Dram Equivelent has never been an indicator for "Max" pressure, only the velocity of the load. Several factors in addition to shot wt & velocity, play important roles in "Max" pressure.
If you fire a given shot wt to a given velocity then the same work has been performed, but the pressure curves can be entirely different, thus a totally different max pressure. The Max pressure is just one small point of that curve. What you gain at one point, you lose elsewhere.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra