Hello Niels,
We've checked the provided transformed ortho, and it seems that disabled color interpolation option confuses you.
For example, without interpolation you have dark green color for the index values approximately in the following interval [-0.08, 0.32] and everything above 0.32 is black, so using 0.1 step will cut off all values above 0.3 and you'll see that both dark green and black areas are in the same polygon.
So I mean that the palette step doesn't correspond to the contour lines steps and is not very detailed when interpolation is switched off. If you turn the interpolation on, you'll see the index representation in more detailed way.