PT100 sampling and accuracy
Limits include the sensor accuracy (calibration and interchangeability at the temperatures of interest), self-heating of the sensor, coupling of the sensor to the thing you're trying to measure, effect of the sensor on the thing you're trying to measure (heat input or heat loss, for example), instrument calibration and changes in that with time and environmental factors, and noise. 

Noise (including quantization noise) is the only one that you can improve by averaging measurements, and not necessarily even that, depending on the type of noise. 

Most errors sources are systematic, so you may get really low-noise measurement of a value that is not very accurate.