I think the GaugeMate 16/20 adapters have a machined 'split' the whole length of the adapter, so they are not rigid enough to completely contain pressure like one of the inserts w/o the split.
It seems that the total radial force from pressure inside the 20 gauge chamber part of the insert is contained by a larger surface at the 16 gauge chamber. If so, then the pressure at the larger surface would be lower than at the smaller to have the same total force. The insert is passive, and it could not add to the force being contained. Does this make any sense?
