I'm not 100% sure either, but the explanation given in http://www.tofugu.com/2012/05/15/japanese-web-design-why-you... seems not too far off.
The bias is quite evident in that article: "One culture's clutter is another culture's useful information." Being from the East myself, I've always found the increasingly "information-averse" nature of the West to be a little odd.
Short version, in a 'high context culture', things can be left unsaid and the cultural background will explain effectively to people within that culture (and less effectively to people outside that culture).
A Texan (the American south is high context) can say quite a bit with 'yep...' and letting the culture explain the rest of the meaning.
In a low context culture, less emphasis is placed on single word choices (though they can still be important) and more explanation of the entire context of the communication is done.
The use of highly cluttered web designs is also a distinctly East Asian phenomenon, but I'm not sure what the cultural explanation for that would be. It's especially odd to see it in Japan, where a more sparse aesthetic dominated the traditional culture.