Comment by fadeyev

The margin under the subheading should be the same (or at the very least not smaller) than the bottom margin of each paragraph, otherwise you end up with the subheadings looking like they're attached to the paragraphs that directly follow them, creating a false hierarchy with a single paragraph rather than a selection of paragraphs. If you want to give each section extra separation increase the margin above the heading instead of decreasing the one below it.