body language is relative to age and gender
Many body language signals are relative.
A gesture by one person in a certain situation can carry far more, or very little meaning, compared to the same gesture used by a different person in a different situation.

Young men for example often display a lot of pronounced gestures because they are naturally energetic, uninhibited and supple.

Older women, relatively, are less energetic, adopt more modest postures, and are prevented by clothing and upbringing from exhibiting very pronounced gestures.

So when assessing body language - especially the strength of signals and meanings - it's important to do so in relative terms, considering the type of person and situation involved.

faking/deception
Some people artificially control their outward body language to give the impression they seek to create at the time.
A confident firm handshake, or direct eye contact, are examples of signals which can be quite easily be 'faked' - usually temporarily, but sometimes more consistently.
However while a degree of faking is possible, it is not possible for someone to control or suppress all outgoing signals.
This is an additional reason to avoid superficial analysis based on isolated signals, and to seek as many indicators as possible, especially subtle clues when suspecting things might not be what they seem. Politicians and manipulative salespeople come to mind for some reason.

Looking for 'micro gestures' (pupils contract, an eyebrow lifts, corner of the mouth twitch) can help identify the true meaning and motive behind one or two strong and potentially false signals.
These micro gestures are very small, difficult to spot and are subconscious, but we cannot control them, hence their usefulness.