It is very common to see 'nominal' values of 'quantities or dimensions' when the real value of that quantity is subject to variation (e.g. when there is natural variation or manufacturing tolerance). However, when that is done, the nominal value quoted is almost invariably some sort of average (mean, median, mode or whatever) of the 'real values' around which the variation exists - and therefore represents the best available indication/estimate of what value is 'expected'. Off the top of my head, I can think of no other situation in which a 'nominal value' is quoted which is clearly appreciably different from any sort of average of the real values that one could think of - and hence which is a poor indicator/estimate of what real values are likely.