Increasing the dX (delta X) to 0.41 works.
However, in my understanding, the slope should be purely defined by the given dX and dY (slope = dY/dX). This means that a rounding error in the dX should not have such a profound effect on the obtained results.
For my example the slope threshold should be 0.001/0.4=0.0025 but I have cases with a slope of 0.2655 which are not selected just because the dX was 0.400085.
