It's that simple. A core percentage of your users simply refuse to read what is in front of them. They skim, they look for only the things they expect, and absolutely nothing else. They're basically running the visual equivalent of an Expect script, responding to a prompt in a way which may or may not be appropriate.
I'll give an example. I had a password reset program which, of course, required an email address specific to our users. However, there was a possibility that users would use a different email address that wasn't valid. No, I had no access to that list of addresses to match and verify, that was simply out. Instructions were provided on the page as to which kinds of email addresses were allowed, and which were not. Of course users ignore instructions, as we all know. They see "email address" and they begin typing. Instructions could be on the top, on the bottom, or right before that row -- it didn't matter. This happened about 45% of the time. Okay, half of our users read directions, yay.
80%? Such an optimist!
Only thirty percent of my users, presented with a HEY IT LOOKS LIKE YOU ARE USING THE WRONG EMAIL ADDRESS HERE IS WHAT IT SHOULD LOOK LIKE kind of prompt, right in their face, changed their behavior. It was big! It was colored! It covered the form until you made it go away. The rest kept doing it the wrong way. Sometimes several times in a row (I watched the logs). I could see them return a month later to reset their password, which they had forgotten, and make the same mistakes. They will not remember more than a few minutes ago, they will not read instructions, and they will not respond to error messages. They will not remember error messages they see.
Once you accept the existence of this core group and realize that you can do nothing to help them besides providing hopefully appropriate environmental prompts, the rest explains itself. Do not waste time trying to solve the problems with this group. If you carefully watch their behavior on other systems, they experience identical problems wherever they may go. They exist in a perpetual struggle with computers, thrashing like a goldfish thrashing in a corner of an aquarium. Feel sorry for them if you must, design around them if you can, but you cannot change their behavior for them, only provide a padded environment where they cannot hurt themselves too much and can hopefully mouth soft objects with rounded corners.