Answer:
uniformity of output
Step-by-step explanation:
Based on the scenario being described within the question it can be said that this difference is in terms of uniformity of output. This term refers to when manufacturing or in this case business operations are smooth and efficient, but output is more variable than not. Which is exactly the case in this scenario since Stephen is able to give various haircuts in a single day making him very efficient, but the quality (output) of those haircuts are not always the same thus they are variable.