When you are using Subdivide Simple, you have the Catmull-Clark option enabled in the modifier. What subdivide simple is doing is creating propagating the detail like in the first example in your video but the subdivision is created with the displacement of your second example on top of that (it would be like if you use regular subdivisions but you sculpt the mesh until you make it look like a perfect cube). So, when you create new geometry, this displacement is distorted and that is what you are seeing. I know that it can be confusing a maybe we will reorganize the options to make this more clear, but the Simple option in the modifier and the Subdivide Simple/Linear operator do not write the same displacement into the subdivisions.