Another complicating factor that many less-tech-literate don't have a good internal model for is window focus. I've seen several people try and single-click on a not focused web button, only for nothing to happen. When they click again, the button is activated. They then learn to always double click that button.

Having a mental model of "this button needs to be double clicked" gets them the result they want, even if that's not a very accurate reflection of the computer.