Netscape 6, based on the Open Source Mozilla platform, does the same thing. Windows' default resolution is no better or worse than Mac resolution. But it is the most commonly used resolution, so more sites are designed to accommodate it. Not so fast, buckos. The new browsers start at this default resolution, but enable users to change it.
Adept users can change it to a size based specifically on their screen resolution.
They can change it to almost anything. Sniff all you want to. Write a thousand Style Sheets and ten thousand levels of conditional branching, and you will still louse up some users if you serve points in your Style Sheet. If you use points to control screen sizes, you are kidding yourself. Pixels are the only reliable means of sizing typography if the designer absolutely must control the size of type on the web page.