Thank you for wanting to help, but,
a) You missed the point. (Look at the original question again)
b) You are mistaken about ISA.
c) We have solved the original question.
ISA is not just a standard pressure, but a standard "atmoshpere", meaning it defines the pressure gradients in that atmosphere. That gradient is the "map" and not the "territory" of reality in the atmoshere on a given day.
We were understanding how they are calculated, and their physical meaning, not their use (with which we are intimately familar).