Pain, but Not the Physical Kind

So I was in the office with Joos the other day talking to my PSG about adjustments on a SAW, M249 for the acronym impaired. I was confused as to why the book referred to the adjustments as mils when I was used to using minutes of angle. My PSG agreed that I was to question it, admitted that he was unsure what they were called and told me exactly how to find the right answer. Joos sort of just watched. This all happened while a 6'2", 300 lb. man was standing uncomfortably close to my back. The part that really makes me nervous is that he had an angry look on his face as if I had done something to him. He's the one that invaded my personal space. Then he tells me that I'm comparing apples and oranges, explaining himself by saying that mils are an angle measurement in land navigation and minutes of angle are for shooting, obviously something I already knew and anyone that makes it through basic would probably know. So I roger my way out of the office as quickly as possible and walk with Joos back to the room where we find out that "mils" is the abbreviation used in the field manual to refer to milimeters. So the standing-too-terribly-close-to-soldiers-pisses-me-off operations sergeant was wrong and was too afraid to admit it.

Why are people afraid to admit that they don't know? It's easier to admit it so later you don't get caught in a lie, as demonstrated above. Thank you PSG for keeping it simple.