Why does this definition for Integral Nonlinearity has -n*Vlsb measured in it? [data converter]

Here's the equation :



For all i know the Integral Nonlinearity is deviation of the values on the actual transfer function from a straight line (ideal transfer function) so the equation should just be a difference between the real value and the ideal one but that's not what is shown in that image. There's a -n*VLSBmeasured 1(why 1?) in it that i dont understand