Aggregation Behavior, Something To Do With MembersWithData Setting But Can't Nail It Down

I have a dimension with a hierarchy. In this hierarchy it is possible for a non leaf member to get loaded with a fact measure.

Using a sales organization as an example, let's say
(1) Bill and Ted are sales reps, and they both report to John.
(2) John, not only manages Bill and Ted's sales, but he also makes sales of his own.
(3) Bill sells 10 units, Ted sells 8 units, and John sells 5 units.

In this example, John is the non-leaf member because he is above Bill and Ted in the hierarchy. When this data is aggregated in the hierarchy, we want to make sure that 10 for Bill plus 8 for Ted do not overwrite the 5 for John. Instead, we want to include the 5 for John with Bill and Ted's total of 18, thus getting a total of 23 for John.

Here is my question. What is the default aggregation behavior? The behavior I get is that the non leaf data is overwritten by the sum of the leaf data. So in the example above, I get 18 for John, not 23.

Now, from my research, I think the MembersWithData setting on a dimension attribute as something to do with this, but I can't nail this down.