ac hoon - You seem to have a duplicate thread going and I responded to the other one! Just in case it's of any help, here is what I put:
Regarding your second question. QNH is calculated by measuring surface pressure and adjusting to sea level based on ISA temperature deviation, and not actual temperature deviation. This is so that when you dial QNH into the altimeter, because it reads relative to ISA conditions, you will get airfield elevation regardless of the actual temperature deviation. This is deliberate so that you can use QNH for instrument precision approaches.
My understanding of the first - it says 'isobaric' surfaces, meaning surfaces with equal pressure. If you had two columns of air at ISA conditions then both surfaces would be at equal height, but if there was a temperature difference then the 'hotter' one would be higher. I'll wait for someone else to give a proper explanation of this as I'm not sure myself if I fully understand the question.
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