Explanation by Vincent Alzieu of Tom's Hardware page.
Some dead (sub) pixels are not easily recognized.
Each pixel is made up of one red, one green and one blue sub-pixel.
A defect in a sub-pixel is not very visible, and can often only be seen against specific backgrounds.
Adjacent sub-pixel defects appearing close in proximity are more visible than â€œgeographically dispersedâ€ defects.
A full-pixel defect (all three R/G/B sub-pixels always on or always off) is quite visible.