The Function called by Doctors and Wasteland Workshop's Decontamination Arch in GenericDoctorsScript.pex to remove all radiation contains a type mismatch. It pulls the current amount of Rads into an Integer, and then uses that as the amount to heal... and dumps the result back into the Rad counter as a Float.
This results in the function failing to heal any fractional Rads present, due to the decimal being truncated when the Rad value is pulled into the Integer storage variable. The remaining fractional Rad causes the player to end up down 1HP from their actual maximum.
The decompiled version of the script I looked at seemed to declare the storage variable on the spot, so it should (hopefully) be an easy fix.