Inconsistent results from cubic interpolation

I'm getting funny output from 'griddata' when using Cubic interpolation technique. I suspect that this is due to the fairly large input coordinates, but I would like to dig to the bottom of this.

Attached file can be used to demonstrate the problem. All I do there is initialize a set of points, and find the interpolated z-value of a single point.

when using the original coordinates, the ouput is

    -4.4466e+05

Now, when i use the same set of points, but moved to be closer to the origin (coordinates of magnitude of tens of thousands rather than billions), the output becomes

    -5.9712e+03

(Or other values, depending on exactly how far the points are moved) The output of the second run is definitely (more) correct than the first one (which is a much lower value than any of the input points!).

Does anyone know what causes this behavior? It only happens for cubic interpolation, all other methods give consistent results.