It seems you are using a proportional font. That means that spaces are narrower than digits. You should use a fixed-width font if you want them to align in this method, e.g. Courier.
This happens due to the font(because the characters don't have the same width) Change the font to a monospaced font and the problem will be solved(I recommend Consolas font for this)
"%3d x %3d kg"would work too.