Why your Posture isn't Perfect
Did you ever notice that images in books or posters about "good posture" are practically always drawings and not photographs?
Perfect Posture is an ideal. Pictures about how to "stand straight" or sit at your computer in an "ergonomically correct" position are fictional.
No One has "Perfect Posture"
Your posture is the practical end result of how you balance your body. Real people don't have "perfect posture" because posture is the result of how imperfect bodies adapt to old injuries and living in the real world.
However, there are better (or stronger) and worse (or weaker) postures. Poorly balanced posture requires more energy to stay upright, causing some areas to carry more stress and wear out more quickly than nature intended.
The good news: Weak posture can be strengthened. Improving posture involves more than just telling someone to stand straight and keep their shoulders back. Improving posture means strengthening how the body balances, and how it moves.