The umpires are also the human beings. They are as normal as we are and they do the same day to day activities that we do. the umpires gives their decision according to their perception. This does not have anything to do with the URDS system.
The URDS system provides the additional support to the umpires so that they can be more accurate in their decision making.
So, according to me we should not compare the URDS system with the umpires, because it is only the additional support provided to the umpires by the means of the technology for getting the correct decisions.