I've come up with a thought experiment. Please point out my error:
Assume zero atmospheric pressure and no boundary effects of the containers. We are measuring the pressure on the green ball. The red line represents a movable barrier.
In fig A and B, the force applied by the top columns of water are unequal. Top column B has more weight, and so the pressure felt by ball B is greater than the pressure felt by ball A.
Fig C and D are as in the first diagram, seemingly ball C and D feel the same pressure.
Since the barrier is movable, systems A and C are identical, as well as B and D.
But B > A = C = D = B.