When we're invited for dinner, we usually want to please our host. This seems true for many cultures, but the way to do this can be quite different.
In China, bringing a gift is a must. Gifts are of course always nice, so you better bring one if you're not sure you should. Although, some gifts are not-done in China, because they may mean bad luck.
Anyways, you brought a gift, the host is happy, he serves you dinner, and then… Should you finish your plate, if you want to be polite? In the Netherlands, we say you should, or you're wasting food. In Thailand, you should not, because it will mean that it wasn't enough. In Japan, not finishing your plate means you didn't like it.
So, maybe you thought that being polite is just logical, but all three options above are based on logical reasoning, but the outcome is quite different!