Explanation:
the median is the value, where half of the other values is smaller (or equal), and the other half bigger (or equal).
with an even number of data points that is not directly possible to find.
in this case the 2 middle values are taken and their mean value is the median.
in our case, the 2 middle values are 12 and 14.
their mean value is
(12+14)/2 = 26/2 = 13
so,
13 is the median (even though if is not a data point itself).