To quote Donald Rumsfeld, there’s known knowns, known unknowns, and unknown unknowns.
In the case of web development as with many things, some people who use web sites, think they know how easy/hard it is to build them. (Hint: they don’t)
Check out this person who needs to hire a programmer yet they already ‘know’ how ‘hard’ the task is and how long it will take. link.