It's because most browsers, out-of-the-box, have a default font size of 16px (which is why we can set base font for a page at 1em and then whittle it all down with 90% and 80% navigation and so on). Most folks never reset that base setting, so many designers will just say "font-size:100%" and let the user's own browsers set the base point upon which the relational fonts get built. The problem is that Safari doesn't have a default font size; all it has is a minimum font size -- and in the absence of a site providing its own default font size, the minimum size is what you get. That would be 9px for me. Ow!

(Which is kinda moot since I've learned not to up the minimum -- b/c then when I'm doing my own designs, I can get duped into thinking "wow, that's plenty large" and not realizing it's because my own base font is affecting the relational font sizing. Made that mistake once, and wow, was that a hard layout to read!)