I suspect it's like how my brain reacts to negative numbers, or decimals; I have no idea how anyone could fail to understand them. But some people do.

And, due to my tendency to analyse mistakes I make (especially factual errors) I remember the times when I got each one of those wrong. I even remember the logic I used.

But they've become so ingrained in my brain now that failure to understand them is nigh inconceivable.