In a symmetric histogram, you have the same number of points to the left and to the right of the median. An example of this is the distribution {1,2,3,4,5}. We have 3 as the median and there are two items below the median (1,2) and two items above the median (4,5).
If we place another number into this distribution, say the number 5, then we have {1,2,3,4,5,5} and we no longer have symmetry. We can fix this by adding in 1 to get {1,1,2,3,4,5,5} and now we have symmetry again. Think of it like having a weight scale. If you add a coin on one side, then you have to add the same weight to the other side to keep balance.
