The built-in ADC has a repeatability of about +/- 3 LSB, which for a 10-bit ADC translate to approximately +/-0.3%.
Note that the reading you see on screen is multiplied by 4 because of the normalisation to 12-bit. i.e. for +/- 3 LSB the reading will fluctuate about +/- 4,8 or 12.
If the reading jump more than that, then ripple on the PLC power supply may play a factor if I assume that your 4-20mA simulator has ultra steady power supply.
If you use the PLC's built-in 5V to power a potentiometer and take measurement the reading will be much more stable.