Odd Interpolation Problem

Hi all,

I am newbei here. I am using excel to calculate some automative tuning related formulas.
Here i have very weird problem. I have attached the excel file.
Actually the interpolation works very well but when I swap the values Y to X it comes up with very odd values.
Why this happens? It should be related to the values I'm entering. Can you please help?

cheers
cuneyt