Mean of random sampling

Hello there, I was wondering if someone could help me with a portion of my stats homework.

Basically in part of the homework I had to take a population of 20 people and take 6 random samples of 5 scores each. I then had to calculate the mean for each sample. Then I put those means into a frequency distribution and calculated the mean and standard deviation of those sample means in the same way I would for any distribution.
So the question is "Does the mean of the sample means equal the population mean exactly?" Well I know that it does not equal the population mean, but I have no idea why. I'm just wondering if there's some specific reason that it does not equal the original population's mean.

If anyone can help me out with this, I would really appreciate it, and thanks in advance.


please attend , because of you dont sample all of possible samples that their size are 5 of population then mean of samples's mean is not equal the population mean , because you dont have all of possible samples .