2 colors-BMP printing doesn't respect the B&W picture

Using PdfCreator1.2.0 (with std gs9.00), a Black & White sketch is badly saved in into a monochrome B&W Bmp file ("2colours"): The original black areas were all transformed in "grids" ("trame" in french).

Result is effectively in B&W pixels but black areas appear now in grey because of this "grid" effect.

Does someone have an idea how to by-pass this, to obtain full black areas ?