Why do we feel first impressions are so accurate?

It is widely held that first impressions are usually correct.  Why is this?  Once we form an initial opinion about someone, we naturally process future observations of that person in ways that support the first or initial impression. And, we unconsciously discard any observation that is in conflict with our initial observation.
I am suggesting that our subsequent experiences of people are simply biased by our first.  If so, then of course our first impressions are always correct.  Later impressions are fueled by the first.
I believe that first impressions are in deed not always accurate.  We are well served to keep an open mind in subsequent early experiences of others.  Rely more on a collection of experiences rather than a single experience.