Since chlorine does vary from time to time and in different points in the system, I would feel sure that there is a lot of "slack" built into the formula for the dechlor product. If I remember correctly, Prime says something about it being safe even at ten times the normal dose. Someone with Prime on hand might check that number for me.
When you are on a county system, I'm sure they are monitored and required to take samples for testing by the state, so the chlorine will never get beyond the high limits but there is some slack built into the limits as it is something that has to vary. Right now there are lots of people in the Northeast who did not prepare as they were warned to do and now they have all these things that they have never asked about. One is the chlorine in their water.