EDIT: @CAI was right, it's not the last-child rule that's to blame for the weird lay-out. It's the fact that text in a li element is not seen as a li-element when the li has a ul nested inside of it. As suggested by @CAI in chat, this could be solved by adding a top-padding to nested ul's.

I think this particular CSS is the culprit:
.post-text ul li, .wmd-preview ul li, .post-text ol li, .wmd-preview ol li {
margin-bottom: .5em;
}
.post-text ul li:last-child, .wmd-preview ul li:last-child, .post-text ol li:last-child, .wmd-preview ol li:last-child {
margin-bottom: 0;
}
So List-items are getting 0.5em margin at the bottom, except for the last one. This makes the last list-items stick to a list item below it when nesting lists or having multiple lists following each other.
outer_garden
andouter_wall
is completely different. The markup looks exactly the same for both but one has<p>
tags added and the other doesn't. Very strange.