Hysteresis is produced when the displayed values are too small for increasing signals and too large for decreasing signals.
This is commonly caused in mechanical instruments by loose gears and linkages and friction. It occurs widely with things involving magnetisation and demagnetisation.
The calibration may be correct at the maximum and minimum values of the range but the graph joining them may not be a straight line (when it ought to be). This is a non linear error.
The instrument may have some adjustments for this and it may be possible to make it correct at mid range as shown.
