Public order is mechanically different. It isn't stacking. It simply stats a point at which no disorder can happen as 100%, and how far above that you are is the margin you have against things like spies, etc. It is a probability issue below 100%, but above it it isn't anymore because the events it is measuring against can't happen anymore. 130% public order is 0% chance of riots/rebellion with a 30% safety margin that has to drop before it can happen.