Well, one is well-defined, easy to measure, boring and not particularly useful. But the other (cruising speed) is more likely to be misunderstood because it's less well-defined and more difficult to measure accurately. Nevertheless, as others have pointed out, countless riders have managed to successfully categorize themselves as being among the more accomplished, typical, or less accomplished road riders according to "average" speeds of approximately 21, 17, and 13 mph respectively, so if it's in our heads, it's in our head the same way that the sky's being blue is in our eyes.