We start from the following idea of counting, where the sets are determined by the ellipses and the units by the "x".
Since each set has 2 units, we have 2 + 2 = 4 units. Now look at the picture below:
Note that in this figure only one set has been added, so there is no addition of units. This means that adding the units of the second figure will be the same as adding the units of the first figure. So we have to 2 + 2 + 2 = 4.
We use the sets referential, but we add in the units referential. There is a mixture of references, which leads to inconsistency.