Home field advantage. They get a sight advantage (studies have shown) that put the odds more in their favor. When sports are down to fractions of an inch, any bias can mean more medals.
When you are the host country, you automatically qualify for events you may not have otherwise. Being the host country means you can enter athletes in at events, and automatically qualify, simply because you are the host.
It is interesting to note, by the way, the end of the article, where they theorize that even though home countries medal counts do go up, the average is about the same, meaning the bump is less meaningful than it seems.