It's probably dictated to some degree by some aspect of culture. When you visit the websites where another culture dominates, one might find things to be different from what we're used to but similar to others where that other culture dominates the design.

A very good point I think, the web in your language is not always the same as the web in other languages, for example sites like nicovideo.js do tend to be different and carry a lot more information per page than American or European sites.

nicovideo.jp, I assume?

An easy mistake to make as s and p are right next to one-another on the qwerty um ... I might not be paying attention.

Because it flows naturally between mobile and desktop. Mobile first design is harder than most of us admit, and mobile first that looks good is even harder.