Wkhtmltopdf - Slope of Chinese characters

I tried to convert http://book.kanunu.org/book4/10406/231373.html to PDF.

I used the following command: wkhtmltopdf --encoding gbk http://book.kanunu.org/book4/10406/231373.html test.pdf

And I got the following result: https://cloud.githubusercontent.com/assets/1232162/3973254/d67340e6-27e5-11e4-9b30-a7df42e0bc41.png

As you can see, the symbols are slanted. I've tested more websites and realized this only happens for Chinese and Japanese characters. Does anyone know why this is so?