Are you asking why the 54.7 degree angle is not 90 degrees?  Whoever designed this part decided that was a good angle. Why do you think 90 degrees would be better?

In a Steady State Thermal analysis, the Degree of Freedom at each node is the Temperature of that node.  That is what the solver is trying to compute, the temperature at each node given the loads and boundary condtions applied at various faces or bodies.