Why is there a difference between the expected value and sample mean?
You would have to sample the entire population in order to get the two equal. Your sample may not be representative of the entire population. For example, suppose the population is 1,1,1,1,1,3,3,3,3,3. The expected average is the population average of 2. If you only sample the first five members, your sample average will be 1. Your sample was not representative of the entire population.
