I think that writers are avid people watchers. We’re most comfortable on the edges of the action where we have the best view.
We notice little nuances of facial expression or body language or turns of phrase, things that other people are completely unaware of as they hustle along their way.
We gather little details and tuck them away for later. I suppose that makes us introverts, turning things over and over in our minds.
So when you’re reading a novel, you shouldn’t be surprised to see things you recognize from time to time. We’re paying attention.