Tolerance Stackup Analysis Lecture - 01 | Kevin Kutto | Designgekz 

I think we will select mean dim...anyhow confused

It will never be same because you are looking at two different things. In the example, the maximum total length of the part is fixed (421.5). No matter what happens, the left and right extremes of the part are defined. And therefore the maximum total length is defined. All the variations will happen to the inner dimensions. In the second case, the total length of the part is not defined. So if you consider the maximum of all the inner dimensions and add them, it will definetly be greater than 421.5.