calculus multiple integrals : im having this exercise in my class but I feel I still cant understand it although my professor explained it to me

[buinam]

The question is list 5 multiple integrals that the same with above
One of the solution is here:

but instead i use E = { (x,y,z) | 0<=x<=1, 0<=y<=x, y<=z<=1} U { (x,y,z) | 0<=x<=1, x<=y<=1, x<=z<=1}
I thought they was the same but it not it give different result after I put f(x,y,z) =1
I can t understand why it different, it seem similiar.