Electronegativity is not a directly measured property. There are several different approaches to assessing it. Look at the first historically. Pauling determined the electronegativity difference for a particular bond, and then arbitrarily assumed the electronegativity value for one element. He determined a lot of such differences and therefore had to average the determined values. According to Pauling, electronegativity is not a value for a specific bond, but for a specific element that allows us to roughly predict some of the properties of chemical compounds.