layout element layout group changes size when changing padding

i have a horizontal layout group of 4 vertical layout groups. i set it to expand children by width, so these 4 groups have same width. but when i set left padding of one of them to 10 then this group has larger width (actually, it is exactly 10px larger). can somebody explain why is this happening? or is there a way to achieve this goal of having 4 vertical layouts with same width, but with different paddings?