Variability

Variability is the reason statistics exists, since identical values would leave nothing to summarize and nothing to infer. Inside a single data set you measure it with the standard deviation or the IQR, which is how two classes can both average 75 while one runs from 70 to 80 and the other from 45 to 100. Across repeated samples it appears as sampling variability, the sample-to-sample bounce in a statistic such as $\bar{x}$ (x-bar), and that bounce shrinks as the sample size grows. Standard error is the name for the size of that second kind of variability.