I had to double check to make sure I responded to the right person after seeing this pop up in my replies summary :D. One way to perk up during morning coffee!

I'm sorry you feel "you didn't understand" has a similar range of interpretation as something like "you have absolutely no idea what you're talking about". I'm not sure I can help with that, I can only say this is not the interpretation of every native English speaker. If anything, the split in views should hopefully help convey English often cares about context and the audience over a singular meaning for a set of words (though I don't think that's particularly unique to English).