Depending on your screen resolution and which program you're using, Moom may not be able to resize a given window to exactly 50% of your screen's width or height. If this happens, it's because the program itself specifies a minimum width and/or height for its window–and Moom cannot resize windows smaller than the program's defined minimums.
So if your screen width is 1280 pixels, and a given program specifies a minimum window width of 800 pixels, then clicking one of Moom's half-width buttons will not get you a 640 pixel wide window. You'll get an 800 pixel wide window, because that's the narrowest allowed by the program in question. We can't "fix" this problem, because it isn't a problem; it's a restriction imposed by the program whose windows you're trying to resize.