Note that numbers in HN (and other) salary surveys can be distorted upwards too; people lie and half-lie on salary surveys, usually in the direction of claiming to be doing better than they really are. And while the HN demographic skews young and inexperienced, it also skews bright and energetic. (Which of course you ought to be too if you're considering consulting or freelancing in the first place.)

So: the numbers can be way too high because of lying, way too low because of demographics, on the high side because of demographics, way too low because it reflects people doing too-generic work, poorly chosen because it's from inexperienced or unsophisticated people, etc., etc., etc.

Better than nothing, no doubt. But use with extreme caution.