Here is the promised explanation why sometimes the interpolated values are not between the given "anchor" values.
Main reason for this is because I used cubic spline interpolation and not just a linear interpolation between two adjacent points only. Usually spline is to be preferred but of course it depends uponb what you would like to achieve and what your data is. If the data is in ascending or descending order the effect won't show either.
For whatever it may be worth I added linear interpolation to the worksheet you find attached.
I have created a 2-dimensional example to demonstrate the difference and how this effect arises.
Spline: f(4.5*10^15) is smaller than both f(4*10^15) and f(5*10^15)
Linear interpolation
Message was edited by: Werner Exinger