Sampling error
From The Glossary of Digital Photography
The developing of conclusions based on a small amount of data and using those conclusions to apply to the entire population. The difference in the conclusions made with the two sets of data is called the sampling error. In digital photography, there is very little light in the darkest shadow areas. Thus the image data collected from those locations tend to generate more noise than in the highlight areas. Part of that noise is due to sampling errors of the few photons available in the dark areas.
