What does the edge look like?
Now, this is just an idle thought of mine—who knows if I'm wrong. But I see that the polka dots in that DMT stone, which are plastic, form a kind of track where the diamond suface is cut and then resumed, cut and then resumed; whereas on the sides of such "track" there are unbroken lines of continuous diamond surface. My take is that this might be creating an uneven sharpening of the edge. My belief is reinforced since you mentioned that you don't have issues with your whetstone, so I think that a continuous abrasive surface, such as the kind you have on a whetstone or a diatoma diamond plate abrades the edge through and through, unlike this patchy DMT plate.
I'm positive that, according to the manufacturer's literature, such polka-dotted surface should have no effect on the edge, but maybe that's because they sell these plates to sharpen knives that are swept at an angle across the plate, rather than from top to bottom—maybe this oblique sweeping reduces the effect of the breaks in the surface—I can't tell, but maybe that's why you get better results when you zig-zag the blade across the stone versus when you run it parallel.