If you tapped out the tune of "Happy Birthday" with your finger on a table while another person was listening, do you think they would recognize the tune?
What if you repeated the experiment 120 times, with 120 different listeners. How many of those 120 people do you think would recognize the tune? If you guessed more than three, you would probably be wrong.
Elizabeth Newton ran just such an experiment at Stanford University in 1990:
[The tappers] expected the listeners would recognize the song about half of the time. Yet of the 120 songs tapped, the listeners only guessed 3 songs correctly. How were the tappers so wrong in their estimate? The problem was that the tappers could hear the melody in their head, but the listeners couldn’t. The tappers’ knowledge of the song caused them to miscalculate the listeners’ comprehension.
It's Not About Being Smart
Intelligence and knowledge are two different things.
Even if you were the world's worst programmer, you likely know more about programming than most small business owners or department managers. (Conversely, those owners and managers likely know more than you about what they do for 40+ hours every week.)
This disconnect often leads to software consultants undervaluing their services.
The thinking goes something like this, "I can't charge X amount of dollars to do thing Y that only takes me two hours to do." The underlying assumption is that anyone could do "thing Y" in only two hours. But not even you, dear reader, could do "thing Y" in only two hours, if not for the hundreds or thousands of hours of learning and experience that you put in to obtain the skill to do "thing Y" in two hours.
When you provide a professional service, the buyer isn't buying (just) a block of your time: they're buying all the time you invested to accumulate the knowledge you have.
After all, "You don't pay the plumber for banging on the pipes. You pay him for knowing where to bang."