Problem
My field contains data from 0 to 100, but the contouring shows values below and above?
Solution
This is an effect of the interpolation, and easy way to get rid of it is to use the 2 following parameters contour_interpolation_floor and contour_interpolation_ceiling,
In this case mcont(contour_interpolation_floor = 0.,contour_interpolation_ceiling=100.). Check the mcont documentation.