ohhh okay I think I get it now. So the point is that we have chosen to project vertical lines as straight instead of horizontal lines as straight (we have to pick one due to projection of a 3D space onto a 2D plane), if we are using monitor match, it applies on x-coordinates irrespective of y values. However, MM does not apply in the y-axis unless x-coordinate is 0. Therefore, choosing MM values other than 0% is going to lead to inconsistencies in y-axis when snapping to targets not aligned to the line x=0.
I hope I am understanding this correctly.