Think about it for a minute or two. I see this mostly watching politicians on the campaign trail. A smile is plastered to a stiff jaw, cold eyes that don't smile and don't meet your eyes, on a head that is held high on a stiff neck with shoulders up and crunched together. Their feet never miss a beat as they walk by you to the next person with their hand out, and the very brief handshake itself is no more alive feeling than that picket fence of your neighbor's you used to run your hands over as you walked by. The politician is going through the expected motions, but would probably prefer to be sitting in a dentist's chair getting a root canal. At least then, he'd have a reason to be tense and defensive, and his body language would match what he's feeling.
Politicians are the most obvious, but there are others equally adept at saying one thing while their body language tells you something entirely different. Others might do it and not even know they're doing it. Still others might do it to tell you what they think you want to hear. Like "I'm fine" when the car just crashed into a telephone pole; and even though you know otherwise due to the obviously broken arm and the pinched, white face, they say it anyway.
Psychology can help us understand this. One term used to describe this is persona. A persona is what a person creates and presents to the world as him/herself. Mostly, it is close to, but not quite, how the person wishes to be perceived. Another term is a façade or mask. It is not unusual for someone to behave differently at home than they do at work, for instance. Another concept is congruence, or simply genuineness. What is happening inside, how one truly feels, is what is present in body language and statements made. What's shown to the world is the same as what is really inside.
What is common in psychology (and it appears to be in politics too) is matter of degree. Adopting a false front may be conscious or unconscious, it may be something that started out as a defense against extreme anxiety or fear and then used all the time, or it may be a way to handle a tense situation with less stress. No matter how prevalent, it seems to be sensed almost immediately. It's worth thinking about, not only while looking more closely at others, but also taking note of how you behave in various situations as well.
This is an introduction and basis for much more of "me, myself and I." As you get to know yourself, really know yourself, you'll be able to understand others more. Oh, and have no fear: This is common among "normal" people.
Thanks for reading! Stay tuned for more...