It is impossible to manufacture anything with absolute precision. In general terms, the more precisely something is manufactured, the more expensive it is. Consider the emonTx current input as a relatively simple example. If we assume for now that we don't have any errors in the sketch, we have 3 physical factors that will each contribute to the uncertainty in the value that we read for the current. These are:
the transfer ratio of the current transformer,
the value of the burden resistor, and
the accuracy with which the burden voltage is measured.