• Flybar is not at horizontal level. (Small difference depends on Mixer ratio setup)
• Blade is not perpendicular to the flybar. (Tiny angle error gives big measure difference)
• Pitch meter is not clamped on the blade in parallel. (Tiny angle error gives big measure difference)

All these tolerance sum up will give over 0.5 degree error of readings, or even 1 degree for some pilots. This is why we normally use another method to measure the pitch.