Assembly (Identity Pairs + Continuity) and Compose Geometry (/Union): Do they represent the same problem?

Dear all,

I am trying a simple example to better understand the Assembly mode. The example is a multilayer (3 layers), where only the layer 2 is at high temperature (so it expands and "pull" the other layers, 1 and 3). The constraints are assimilated to a cantilever beam (the left extremity fixed and the right one free)
I have simulated these configuration:

1. Union mode
2. Compose (Geometry > Boolean operations) applied to the 3 layers, thus independently of the use of assembly or union
3. Compose (Geometry > Boolean operations) applied to the 2 layers (2nd and 3rd), and Assembly mode + Identity pair (1) + Continuity (in Solid Mechanics > Pairs), to couple the 1st layer to the 2nd+3rd
4. Assembly + Identity Pairs (2) + Continuity applied to the 3 layers

Only the results with 1. and 2. are identical, the others are different (as in the pictures that I attached). Why don't all the 4 models give the same result? They should represent the same problem but implemented in different ways in Comsol, or I'm doing wrong?

Thank you in advance!