Here I've made the paragraphs 50 pixels high and turned their borders on so you can see what fifty pixels looks like.

The top margin of each paragraph is 50 pixels and the bottom margin is 30 pixels, so you might expect 80 pixels of space between them.

As you can see, the margin is equal to the paragraph height - 50 pixels; the margins have collapsed and only the larger of the two touching margins has effect.