It seems to be as I remember: I purposely put only "sans-serif" as the font. So if your browser defaults to MS Gothic as the fallback sans font that's what you get.
The idea was that people have different taste in fonts, and this way I won't override the default fonts they have set. But perhaps that's not a good idea today? If almost all sites set a font of their own, then the default fallback may be quite bad in practice.
Perhaps time for an overhaul of the style in general.