Desired roll/pitch vs Actual roll/pitch

Can somebody please put light on what can cause difference in desired roll/pitch and actual roll/pitch values.

Consider that theres no mechanical issue and all servos and actuator mechanism is working fine

Basically poorly tuned (or never tuned, flying with default PIDs) vehicles (PID values) struggle to follow desired roll and pitch angles.

Thank you.

I am flying talon 1.78m wingspan. My roll is fine but desired pitch is not matching with actual pitch.

It will try to desired pitch but suddenly actual pitch will jump to higher values. You can see in this pic