One, we're very quick decision makers. Once we've made a decision about something, we stop scanning, right? We've made our minds up, we stop scanning. So we're not processing anymore. And secondly, it's about experience and expectations. We've never experienced him looking like this, and so we don't expect it.
Reading this I find where I have been at fault many times as a student and as a member of society. It is the classic "never judge a book by it's cover," and I think that more often than not, people find themselves assuming before they reach past the surface of an individual. Someone does a similar action or has a similar characteristics to someone they've disliked in the past and immediately a red flag is thrown. Very unfortunate, and it is important that while it is important to learn from past mistakes and experiences, we must keep an open mind and understand that there is more than just what we know as individuals.