Your assumptions are wrong. The GetById method is supposed to return a single Organization entity. And I want this entity to come from my existingOrganizations list. This is why I'm arranging it with ReturnsMany() and not with Returns().
Taken form JustMock documentation:
"The ReturnsMany method will arrange certain function/property to sequentially return the members of a predefined array (returnValues)."
So my expectation is that every time I hit GetById() in my test, I will get a different entity from the list. But this is not happening, because the whole list gets returned instead.