Why the lower bed lay's thickness decrease when the top bed layer still exist

In this simple example “try_beds.zip”, I build two layers of sediment bed using uniform bed builder. However, when looking to the results, I find that the top bed layer(Layer 2, contact with water column) always exist (B.jpg). Then why layer one, which is below the layer 2, its thickness can change (A.jpg)? In my understanding, only when Layer 2 is erosed totally (its 0.15m is erosed in this example), Layer 1 will change its thickness. Why the two layers’ thickness change at the same time?

Besides, I have set "Max. Layer thickness in Morph&Consol” as a big value (51m), to avoid the additional two layers EE will add. So in this example, the layers are always “1 & 2” which is added originally. Layers 3 & 4 are not activated (you can see in VierPlan). I think this can make my question more clear. Hope for your advice.

Sorry, “Max. Layer thickness” is 0.0001. [Though I use this value in another model and that model crashed. Because in that model, “Top layer” of sediment is 2 @ time 0, while one time step later, “Top layer” became 1. I also confused about the “disappearance” of upppermost layer “2” in the first one time step]