Calling it delusion is too severe. Our reasoning fails us all the time. For example, if you get a problem wrong on a math test, you’ve simply made a mistake, and with a little more “prompting” (even of your own volition) you could often easily arrive at the correct answer. Now if you stubbornly persist with the wrong answer even in the face of evidence to the contrary, then you could call it delusion. But my point is, there’s a spectrum of mistakenness.